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We have two Buffalo WZR-HP-G300NH2 routers, with the default firmware, DD-WRT v24SP2-MULTI (10/31/11) std - build 17798. Each has a separate cable internet connection with a public static IP address. They are both in the 24.123.68.0/24 space.

Both of them can contact pretty much the whole internet, and they can both be accessed out on the internet with no problem, but for some reason they can't talk to each other! When I try to ping one from the other I always get "Destination Host Unreachable". There are no strange routing or firewall rules in place. And they are both set to respond to pings, I can ping them from outside.

Our main IT guy is going to call our ISP on Monday, but I'm impatient, so does anyone have any ideas?

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2 Answers 2

up vote 3 down vote accepted

It sounds like they have their netmasks set improperly. If they're both in 24.123.68.0/24 and they both have their netmask set to /24, then each one will think the other one's hosts are on its local network, so it will use ARP to try to connect to them locally, which won't work.

It sounds like whoever configured the routers didn't correctly set the netmask. It's also possible that individual machines on the LAN have the LAN netmask set incorrectly. Check the routing table on the machines to make sure they're correct to use the router to reach machines on the other network. Then check the routers to make sure the netmasks are correct to use the Internet to reach machines on the other network. I'll bet you one or both of those is incorrect.

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Thanks man! I changed the netmask on the WAN interface from 255.255.255.0 to 255.255.255.255, and it works now! –  HOLOGRAPHICpizza Apr 14 '12 at 22:39
    
That's incorrect, too... with a /32 netmask, they should not even be able to reach their default gateway (i.e. their ISP's remote endpoint). –  Massimo Apr 14 '12 at 23:40

It sounds like you don't have a route between the two.

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