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When I ping, reply is from wrong address

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CADGraphicsIT

Technical User
Apr 7, 2004
28
US
When i ping a certain IP address, the reply comes back from a different address! Anyone ever seen this before and know what's going on?

-Ryan-
Network/Systems Administrator
CAD Graphics West Inc.
 
I've seen this in a couple of different situations:

1. When NAT is employed between you and the destination node.

2. When there is some form of load-balancing occuring between you and the destination node and the destination node has multiple network connections (i.e. IP addresses).

Depending whether 1 or 2 is the likely cause is sometimes simple to tell.

If 1, likely the address returned is vastly different from the one you ping (i.e. if you ping 212.44.60.10 for example, you may get a response from 10.10.2.1 - it's NAT'ed address).

If 2, likely the address returned will be different only in the host bits (i.e. if you ping 212.44.60.10 for example, you may get a response from 212.44.60.21 - a second NIC in the same server but on the same IP subnet).

Hope this helps
 
This frequently happens with computers that are on two networks simultaneously. When the computer attempts to respond to you, it looks for the lowest 'cost' route, and replies on that NIC with that NIC's address. Depending on how the network is laid out, what it considers to be the lowest cost route, and what you consider to be the lowest cost route may be different. Especially if your network is undefined in that machine's routing table, in which case it will choose its default gateway (and the NIC attached to that network) as the return path.


pansophic
 
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