Can I announce prefix 161.117.25.0/24 even though I don't have all of /24 IPs?












5















I'm managing one BGP router, under the router I'm trying to announce the following IP prefix:161.117.25.0/24. But the problem is that some of the IP addresses were borrowed by other AS(Autonomous System), for example the IP address 161.117.25.100 and 161.117.25.200 were borrowed by other AS. So how should I announce the remaining IP addresses please.



Following questions please:




  1. Can I announce 161.117.25.0/30?

  2. Is it possible that two Autonomous Systems announce or have the same IP prefix?










share|improve this question




















  • 2





    ISPs will only advertise /24 or shorter IPv4 prefixes. You need to ask your ISP if you can announce a longer prefix to it, but it will not advertise it to the Internet. No, two ASes advertising the same prefix will break things, and likely get both blocked.

    – Ron Maupin
    Feb 26 at 21:13











  • Is the prefix yours or not? It does not appear to be publicly assigned to you. It should go without saying that if the prefix is not yours, you can't announce it.

    – David Schwartz
    Feb 27 at 0:34


















5















I'm managing one BGP router, under the router I'm trying to announce the following IP prefix:161.117.25.0/24. But the problem is that some of the IP addresses were borrowed by other AS(Autonomous System), for example the IP address 161.117.25.100 and 161.117.25.200 were borrowed by other AS. So how should I announce the remaining IP addresses please.



Following questions please:




  1. Can I announce 161.117.25.0/30?

  2. Is it possible that two Autonomous Systems announce or have the same IP prefix?










share|improve this question




















  • 2





    ISPs will only advertise /24 or shorter IPv4 prefixes. You need to ask your ISP if you can announce a longer prefix to it, but it will not advertise it to the Internet. No, two ASes advertising the same prefix will break things, and likely get both blocked.

    – Ron Maupin
    Feb 26 at 21:13











  • Is the prefix yours or not? It does not appear to be publicly assigned to you. It should go without saying that if the prefix is not yours, you can't announce it.

    – David Schwartz
    Feb 27 at 0:34
















5












5








5








I'm managing one BGP router, under the router I'm trying to announce the following IP prefix:161.117.25.0/24. But the problem is that some of the IP addresses were borrowed by other AS(Autonomous System), for example the IP address 161.117.25.100 and 161.117.25.200 were borrowed by other AS. So how should I announce the remaining IP addresses please.



Following questions please:




  1. Can I announce 161.117.25.0/30?

  2. Is it possible that two Autonomous Systems announce or have the same IP prefix?










share|improve this question
















I'm managing one BGP router, under the router I'm trying to announce the following IP prefix:161.117.25.0/24. But the problem is that some of the IP addresses were borrowed by other AS(Autonomous System), for example the IP address 161.117.25.100 and 161.117.25.200 were borrowed by other AS. So how should I announce the remaining IP addresses please.



Following questions please:




  1. Can I announce 161.117.25.0/30?

  2. Is it possible that two Autonomous Systems announce or have the same IP prefix?







router bgp






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Feb 27 at 6:31









Community

1




1










asked Feb 26 at 19:02









JackJack

1857




1857








  • 2





    ISPs will only advertise /24 or shorter IPv4 prefixes. You need to ask your ISP if you can announce a longer prefix to it, but it will not advertise it to the Internet. No, two ASes advertising the same prefix will break things, and likely get both blocked.

    – Ron Maupin
    Feb 26 at 21:13











  • Is the prefix yours or not? It does not appear to be publicly assigned to you. It should go without saying that if the prefix is not yours, you can't announce it.

    – David Schwartz
    Feb 27 at 0:34
















  • 2





    ISPs will only advertise /24 or shorter IPv4 prefixes. You need to ask your ISP if you can announce a longer prefix to it, but it will not advertise it to the Internet. No, two ASes advertising the same prefix will break things, and likely get both blocked.

    – Ron Maupin
    Feb 26 at 21:13











  • Is the prefix yours or not? It does not appear to be publicly assigned to you. It should go without saying that if the prefix is not yours, you can't announce it.

    – David Schwartz
    Feb 27 at 0:34










2




2





ISPs will only advertise /24 or shorter IPv4 prefixes. You need to ask your ISP if you can announce a longer prefix to it, but it will not advertise it to the Internet. No, two ASes advertising the same prefix will break things, and likely get both blocked.

– Ron Maupin
Feb 26 at 21:13





ISPs will only advertise /24 or shorter IPv4 prefixes. You need to ask your ISP if you can announce a longer prefix to it, but it will not advertise it to the Internet. No, two ASes advertising the same prefix will break things, and likely get both blocked.

– Ron Maupin
Feb 26 at 21:13













Is the prefix yours or not? It does not appear to be publicly assigned to you. It should go without saying that if the prefix is not yours, you can't announce it.

– David Schwartz
Feb 27 at 0:34







Is the prefix yours or not? It does not appear to be publicly assigned to you. It should go without saying that if the prefix is not yours, you can't announce it.

– David Schwartz
Feb 27 at 0:34












3 Answers
3






active

oldest

votes


















16














That /24 is part of a larger block (161.117.0.0/16) which is assigned to Alibabacom Singapore and announced by AS45102 (which is Alibaba again). My guess is that you're a customer of them and have been assigned a number of IP addresses to use from that /24. However, that does not give you the right to advertise them under your own ASN in BGP. If you do so, it would be considered a BGP Hijack.



Hopefully, it wouldn't work very well due to filters in place at upstream networks, but if it would, IP addresses in that /24 used by others would become unreachable.



The conclusion here is: you don't announce the /24 unless you're authorized to do so.






share|improve this answer































    4















    Is it possible that two Autonomous Systems announce or have the same ip prefix?




    It is entirely possible for multiple ASNs to announce the same prefix -- the shortest AS-PATH from any given perspective "wins". However, it's a poor practice that will trigger warnings in any BGP monitoring systems. (see also: route hijacking) If AS1 and AS2 both announce the same prefix, those closest to AS1 will prefer AS1; those closest to AS2 will prefer AS2. As long as you are connected to both ASNs it will usually work, but you've create a very fragile arrangement.



    Beyond BGP (once routes are in the FIB), IP routing uses a longest-match algorithm. The /32 routes for 100 and 200 will match before the /24 (which will match before the /16, etc.) As others have already said, one cannot announce [BGP] anything more specific than /24 globally, so it would be up to you and/or your ISP(s) to make sure those two specific addresses are handled appropriately.






    share|improve this answer































      3














      Just to be concrete, you advertise the blocks you do have. Given a "/24 without .100 and .200", the minimal number of blocks is 14:



      A.B.C.0/26
      A.B.C.64/27
      A.B.C.96/30
      A.B.C.101/32
      A.B.C.102/31
      A.B.C.104/29
      A.B.C.112/28
      A.B.C.128/26
      A.B.C.192/29
      A.B.C.201/32
      A.B.C.202/31
      A.B.C.204/30
      A.B.C.208/28
      A.B.C.224/27


      enter image description here



      Whether your ISP or other organisation will accept such long prefixes is a matter to take up with them: most ISPs have a limit shorter than /24/



      Other, non-minimal, groupings are also possible in principle, up to the limiting case of 254 blocks each /32.






      share|improve this answer


























      • If 161.117.25.0/24 is assigned to a LAN, some trickery would be necessary to put 100 and 200 somewhere else. (I've done this very thing.) From a route table perspective, 161.117.25.0/24, 161.117.25.100/32, 161.117.25.200/32 do not create a conflict.

        – Ricky Beam
        Feb 27 at 0:21











      • @RickyBeam ... from the point of view of routes, certainly. As for advertising blocks across organisations, surely you only say what you actually have, per the other answers.

        – jonathanjo
        Feb 27 at 14:55











      • He announces the /24 and the others announce their /32. Of course, one can't globally route a /32, so he who has the /24 has to rig up a way to deliver the two /32's. (just like how the ISP announces a /16 to the world, and internally figures out how to divide it up)

        – Ricky Beam
        Feb 27 at 18:07











      • My understanding of the question is that the upstream ISP is cutting out the two /32s, not the owner of the nearly-/24.

        – jonathanjo
        Feb 27 at 21:09












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      3 Answers
      3






      active

      oldest

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      3 Answers
      3






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      16














      That /24 is part of a larger block (161.117.0.0/16) which is assigned to Alibabacom Singapore and announced by AS45102 (which is Alibaba again). My guess is that you're a customer of them and have been assigned a number of IP addresses to use from that /24. However, that does not give you the right to advertise them under your own ASN in BGP. If you do so, it would be considered a BGP Hijack.



      Hopefully, it wouldn't work very well due to filters in place at upstream networks, but if it would, IP addresses in that /24 used by others would become unreachable.



      The conclusion here is: you don't announce the /24 unless you're authorized to do so.






      share|improve this answer




























        16














        That /24 is part of a larger block (161.117.0.0/16) which is assigned to Alibabacom Singapore and announced by AS45102 (which is Alibaba again). My guess is that you're a customer of them and have been assigned a number of IP addresses to use from that /24. However, that does not give you the right to advertise them under your own ASN in BGP. If you do so, it would be considered a BGP Hijack.



        Hopefully, it wouldn't work very well due to filters in place at upstream networks, but if it would, IP addresses in that /24 used by others would become unreachable.



        The conclusion here is: you don't announce the /24 unless you're authorized to do so.






        share|improve this answer


























          16












          16








          16







          That /24 is part of a larger block (161.117.0.0/16) which is assigned to Alibabacom Singapore and announced by AS45102 (which is Alibaba again). My guess is that you're a customer of them and have been assigned a number of IP addresses to use from that /24. However, that does not give you the right to advertise them under your own ASN in BGP. If you do so, it would be considered a BGP Hijack.



          Hopefully, it wouldn't work very well due to filters in place at upstream networks, but if it would, IP addresses in that /24 used by others would become unreachable.



          The conclusion here is: you don't announce the /24 unless you're authorized to do so.






          share|improve this answer













          That /24 is part of a larger block (161.117.0.0/16) which is assigned to Alibabacom Singapore and announced by AS45102 (which is Alibaba again). My guess is that you're a customer of them and have been assigned a number of IP addresses to use from that /24. However, that does not give you the right to advertise them under your own ASN in BGP. If you do so, it would be considered a BGP Hijack.



          Hopefully, it wouldn't work very well due to filters in place at upstream networks, but if it would, IP addresses in that /24 used by others would become unreachable.



          The conclusion here is: you don't announce the /24 unless you're authorized to do so.







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered Feb 26 at 19:15









          Teun VinkTeun Vink

          12k53154




          12k53154























              4















              Is it possible that two Autonomous Systems announce or have the same ip prefix?




              It is entirely possible for multiple ASNs to announce the same prefix -- the shortest AS-PATH from any given perspective "wins". However, it's a poor practice that will trigger warnings in any BGP monitoring systems. (see also: route hijacking) If AS1 and AS2 both announce the same prefix, those closest to AS1 will prefer AS1; those closest to AS2 will prefer AS2. As long as you are connected to both ASNs it will usually work, but you've create a very fragile arrangement.



              Beyond BGP (once routes are in the FIB), IP routing uses a longest-match algorithm. The /32 routes for 100 and 200 will match before the /24 (which will match before the /16, etc.) As others have already said, one cannot announce [BGP] anything more specific than /24 globally, so it would be up to you and/or your ISP(s) to make sure those two specific addresses are handled appropriately.






              share|improve this answer




























                4















                Is it possible that two Autonomous Systems announce or have the same ip prefix?




                It is entirely possible for multiple ASNs to announce the same prefix -- the shortest AS-PATH from any given perspective "wins". However, it's a poor practice that will trigger warnings in any BGP monitoring systems. (see also: route hijacking) If AS1 and AS2 both announce the same prefix, those closest to AS1 will prefer AS1; those closest to AS2 will prefer AS2. As long as you are connected to both ASNs it will usually work, but you've create a very fragile arrangement.



                Beyond BGP (once routes are in the FIB), IP routing uses a longest-match algorithm. The /32 routes for 100 and 200 will match before the /24 (which will match before the /16, etc.) As others have already said, one cannot announce [BGP] anything more specific than /24 globally, so it would be up to you and/or your ISP(s) to make sure those two specific addresses are handled appropriately.






                share|improve this answer


























                  4












                  4








                  4








                  Is it possible that two Autonomous Systems announce or have the same ip prefix?




                  It is entirely possible for multiple ASNs to announce the same prefix -- the shortest AS-PATH from any given perspective "wins". However, it's a poor practice that will trigger warnings in any BGP monitoring systems. (see also: route hijacking) If AS1 and AS2 both announce the same prefix, those closest to AS1 will prefer AS1; those closest to AS2 will prefer AS2. As long as you are connected to both ASNs it will usually work, but you've create a very fragile arrangement.



                  Beyond BGP (once routes are in the FIB), IP routing uses a longest-match algorithm. The /32 routes for 100 and 200 will match before the /24 (which will match before the /16, etc.) As others have already said, one cannot announce [BGP] anything more specific than /24 globally, so it would be up to you and/or your ISP(s) to make sure those two specific addresses are handled appropriately.






                  share|improve this answer














                  Is it possible that two Autonomous Systems announce or have the same ip prefix?




                  It is entirely possible for multiple ASNs to announce the same prefix -- the shortest AS-PATH from any given perspective "wins". However, it's a poor practice that will trigger warnings in any BGP monitoring systems. (see also: route hijacking) If AS1 and AS2 both announce the same prefix, those closest to AS1 will prefer AS1; those closest to AS2 will prefer AS2. As long as you are connected to both ASNs it will usually work, but you've create a very fragile arrangement.



                  Beyond BGP (once routes are in the FIB), IP routing uses a longest-match algorithm. The /32 routes for 100 and 200 will match before the /24 (which will match before the /16, etc.) As others have already said, one cannot announce [BGP] anything more specific than /24 globally, so it would be up to you and/or your ISP(s) to make sure those two specific addresses are handled appropriately.







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered Feb 27 at 0:15









                  Ricky BeamRicky Beam

                  22.3k23264




                  22.3k23264























                      3














                      Just to be concrete, you advertise the blocks you do have. Given a "/24 without .100 and .200", the minimal number of blocks is 14:



                      A.B.C.0/26
                      A.B.C.64/27
                      A.B.C.96/30
                      A.B.C.101/32
                      A.B.C.102/31
                      A.B.C.104/29
                      A.B.C.112/28
                      A.B.C.128/26
                      A.B.C.192/29
                      A.B.C.201/32
                      A.B.C.202/31
                      A.B.C.204/30
                      A.B.C.208/28
                      A.B.C.224/27


                      enter image description here



                      Whether your ISP or other organisation will accept such long prefixes is a matter to take up with them: most ISPs have a limit shorter than /24/



                      Other, non-minimal, groupings are also possible in principle, up to the limiting case of 254 blocks each /32.






                      share|improve this answer


























                      • If 161.117.25.0/24 is assigned to a LAN, some trickery would be necessary to put 100 and 200 somewhere else. (I've done this very thing.) From a route table perspective, 161.117.25.0/24, 161.117.25.100/32, 161.117.25.200/32 do not create a conflict.

                        – Ricky Beam
                        Feb 27 at 0:21











                      • @RickyBeam ... from the point of view of routes, certainly. As for advertising blocks across organisations, surely you only say what you actually have, per the other answers.

                        – jonathanjo
                        Feb 27 at 14:55











                      • He announces the /24 and the others announce their /32. Of course, one can't globally route a /32, so he who has the /24 has to rig up a way to deliver the two /32's. (just like how the ISP announces a /16 to the world, and internally figures out how to divide it up)

                        – Ricky Beam
                        Feb 27 at 18:07











                      • My understanding of the question is that the upstream ISP is cutting out the two /32s, not the owner of the nearly-/24.

                        – jonathanjo
                        Feb 27 at 21:09
















                      3














                      Just to be concrete, you advertise the blocks you do have. Given a "/24 without .100 and .200", the minimal number of blocks is 14:



                      A.B.C.0/26
                      A.B.C.64/27
                      A.B.C.96/30
                      A.B.C.101/32
                      A.B.C.102/31
                      A.B.C.104/29
                      A.B.C.112/28
                      A.B.C.128/26
                      A.B.C.192/29
                      A.B.C.201/32
                      A.B.C.202/31
                      A.B.C.204/30
                      A.B.C.208/28
                      A.B.C.224/27


                      enter image description here



                      Whether your ISP or other organisation will accept such long prefixes is a matter to take up with them: most ISPs have a limit shorter than /24/



                      Other, non-minimal, groupings are also possible in principle, up to the limiting case of 254 blocks each /32.






                      share|improve this answer


























                      • If 161.117.25.0/24 is assigned to a LAN, some trickery would be necessary to put 100 and 200 somewhere else. (I've done this very thing.) From a route table perspective, 161.117.25.0/24, 161.117.25.100/32, 161.117.25.200/32 do not create a conflict.

                        – Ricky Beam
                        Feb 27 at 0:21











                      • @RickyBeam ... from the point of view of routes, certainly. As for advertising blocks across organisations, surely you only say what you actually have, per the other answers.

                        – jonathanjo
                        Feb 27 at 14:55











                      • He announces the /24 and the others announce their /32. Of course, one can't globally route a /32, so he who has the /24 has to rig up a way to deliver the two /32's. (just like how the ISP announces a /16 to the world, and internally figures out how to divide it up)

                        – Ricky Beam
                        Feb 27 at 18:07











                      • My understanding of the question is that the upstream ISP is cutting out the two /32s, not the owner of the nearly-/24.

                        – jonathanjo
                        Feb 27 at 21:09














                      3












                      3








                      3







                      Just to be concrete, you advertise the blocks you do have. Given a "/24 without .100 and .200", the minimal number of blocks is 14:



                      A.B.C.0/26
                      A.B.C.64/27
                      A.B.C.96/30
                      A.B.C.101/32
                      A.B.C.102/31
                      A.B.C.104/29
                      A.B.C.112/28
                      A.B.C.128/26
                      A.B.C.192/29
                      A.B.C.201/32
                      A.B.C.202/31
                      A.B.C.204/30
                      A.B.C.208/28
                      A.B.C.224/27


                      enter image description here



                      Whether your ISP or other organisation will accept such long prefixes is a matter to take up with them: most ISPs have a limit shorter than /24/



                      Other, non-minimal, groupings are also possible in principle, up to the limiting case of 254 blocks each /32.






                      share|improve this answer















                      Just to be concrete, you advertise the blocks you do have. Given a "/24 without .100 and .200", the minimal number of blocks is 14:



                      A.B.C.0/26
                      A.B.C.64/27
                      A.B.C.96/30
                      A.B.C.101/32
                      A.B.C.102/31
                      A.B.C.104/29
                      A.B.C.112/28
                      A.B.C.128/26
                      A.B.C.192/29
                      A.B.C.201/32
                      A.B.C.202/31
                      A.B.C.204/30
                      A.B.C.208/28
                      A.B.C.224/27


                      enter image description here



                      Whether your ISP or other organisation will accept such long prefixes is a matter to take up with them: most ISPs have a limit shorter than /24/



                      Other, non-minimal, groupings are also possible in principle, up to the limiting case of 254 blocks each /32.







                      share|improve this answer














                      share|improve this answer



                      share|improve this answer








                      edited Feb 26 at 21:46

























                      answered Feb 26 at 21:40









                      jonathanjojonathanjo

                      12k1937




                      12k1937













                      • If 161.117.25.0/24 is assigned to a LAN, some trickery would be necessary to put 100 and 200 somewhere else. (I've done this very thing.) From a route table perspective, 161.117.25.0/24, 161.117.25.100/32, 161.117.25.200/32 do not create a conflict.

                        – Ricky Beam
                        Feb 27 at 0:21











                      • @RickyBeam ... from the point of view of routes, certainly. As for advertising blocks across organisations, surely you only say what you actually have, per the other answers.

                        – jonathanjo
                        Feb 27 at 14:55











                      • He announces the /24 and the others announce their /32. Of course, one can't globally route a /32, so he who has the /24 has to rig up a way to deliver the two /32's. (just like how the ISP announces a /16 to the world, and internally figures out how to divide it up)

                        – Ricky Beam
                        Feb 27 at 18:07











                      • My understanding of the question is that the upstream ISP is cutting out the two /32s, not the owner of the nearly-/24.

                        – jonathanjo
                        Feb 27 at 21:09



















                      • If 161.117.25.0/24 is assigned to a LAN, some trickery would be necessary to put 100 and 200 somewhere else. (I've done this very thing.) From a route table perspective, 161.117.25.0/24, 161.117.25.100/32, 161.117.25.200/32 do not create a conflict.

                        – Ricky Beam
                        Feb 27 at 0:21











                      • @RickyBeam ... from the point of view of routes, certainly. As for advertising blocks across organisations, surely you only say what you actually have, per the other answers.

                        – jonathanjo
                        Feb 27 at 14:55











                      • He announces the /24 and the others announce their /32. Of course, one can't globally route a /32, so he who has the /24 has to rig up a way to deliver the two /32's. (just like how the ISP announces a /16 to the world, and internally figures out how to divide it up)

                        – Ricky Beam
                        Feb 27 at 18:07











                      • My understanding of the question is that the upstream ISP is cutting out the two /32s, not the owner of the nearly-/24.

                        – jonathanjo
                        Feb 27 at 21:09

















                      If 161.117.25.0/24 is assigned to a LAN, some trickery would be necessary to put 100 and 200 somewhere else. (I've done this very thing.) From a route table perspective, 161.117.25.0/24, 161.117.25.100/32, 161.117.25.200/32 do not create a conflict.

                      – Ricky Beam
                      Feb 27 at 0:21





                      If 161.117.25.0/24 is assigned to a LAN, some trickery would be necessary to put 100 and 200 somewhere else. (I've done this very thing.) From a route table perspective, 161.117.25.0/24, 161.117.25.100/32, 161.117.25.200/32 do not create a conflict.

                      – Ricky Beam
                      Feb 27 at 0:21













                      @RickyBeam ... from the point of view of routes, certainly. As for advertising blocks across organisations, surely you only say what you actually have, per the other answers.

                      – jonathanjo
                      Feb 27 at 14:55





                      @RickyBeam ... from the point of view of routes, certainly. As for advertising blocks across organisations, surely you only say what you actually have, per the other answers.

                      – jonathanjo
                      Feb 27 at 14:55













                      He announces the /24 and the others announce their /32. Of course, one can't globally route a /32, so he who has the /24 has to rig up a way to deliver the two /32's. (just like how the ISP announces a /16 to the world, and internally figures out how to divide it up)

                      – Ricky Beam
                      Feb 27 at 18:07





                      He announces the /24 and the others announce their /32. Of course, one can't globally route a /32, so he who has the /24 has to rig up a way to deliver the two /32's. (just like how the ISP announces a /16 to the world, and internally figures out how to divide it up)

                      – Ricky Beam
                      Feb 27 at 18:07













                      My understanding of the question is that the upstream ISP is cutting out the two /32s, not the owner of the nearly-/24.

                      – jonathanjo
                      Feb 27 at 21:09





                      My understanding of the question is that the upstream ISP is cutting out the two /32s, not the owner of the nearly-/24.

                      – jonathanjo
                      Feb 27 at 21:09


















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