The Cisco ISR 4321 router is not giving Internet connections to the LAN PCs when the WAN interface has a /30 mask length (mask of, which is the actual IP address range I got from the ISP for my leased line. It has no problem using a /24 mask length (mask of

Why is this happening?

  • 1
    What exactly is your question? Why did the ISP use a /30 on the WAN link? (because the only thing on the link is your router and the isp router, and they don't want to use /31's) What address(es) are you supposed to use on the LAN? (whatever block the ISP assigned to you, or none: NAT to the WAN IP)
    – Ricky
    Oct 26, 2015 at 21:32
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    – YLearn
    Oct 27, 2015 at 2:01

1 Answer 1


It sounds like even though that's your block assigned to use the ISP hasn't segmented it out so the ARP broadcasts are working improperly to resolve, this is usually done by smaller ISP in order to conserve IP space. If they were to give an actual /30 to every customer they would be losing 3/4 of the IP addresses they have to allocate (their router's IP, network address, broadcast address). I would confirm that with them, but I've seen it before. Otherwise if that's not the case you can sterilize your configuration and post it so unique problems can be dissected.

  • I could never figure out why ISPs don't use /31 for the connections; they all seem afraid of it. The RFC for using /31 has been around for 15 years! That's several generations in Internet time. Using /31 would give no waste for IPv4 address blocks. They also want to use /126 for IPv6, but that RFC has been obsolete for years, and the current RFC says to use /127. Granted, IPv6 doesn't have the address shortage that IPv4 has, but they should actually use what the RFCs say is correct.
    – Ron Maupin
    Oct 26, 2015 at 14:14
  • I think the reason they don't use it is for equipment restrictions, while there are many devices that support /31 addressing, some don't. Therefore to accommodate customers they do this. I'm sure there are other reasons though. A lot of provider actually use /32s for PPPoE (networkengineering.stackexchange.com/questions/607/…) but that's only for a specific type of connection. For IPv6 I really think it's all about management and uniformity. I don't do much in the ISP space but I can see some valid reasons.
    – Fallacy11
    Oct 26, 2015 at 14:29
  • I haven't run into any equipment in a very long time where /31 doesn't work. If I did, it would be rejected as non-compliant with IETF standards. As far as IPv6, the /126 was rejected because of the "Ping Pong" attack, and using /127 makes a link immune to that particular problem.
    – Ron Maupin
    Oct 26, 2015 at 14:35
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    It was ported into Cisco code in 12.2T, I know for a fact there are devices out there which are not updated to that. There are also a plethora of consumer devices which do not. And devices don't have to be RFC complaint, how many devices support RFC 5549?
    – Fallacy11
    Oct 26, 2015 at 15:34

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