overlapping networks

[carl_townshend]

Can anyone tell me if this will work.

I have a subnet on 172.30/16 and I want to route to the 172.30.200/24 network, will the pc's on the /16 network look locally for the /24 network or will they look at there default gateway for that network ?

thanks

[pkhatri]

Carl,

Because the 172.30/16 network includes the 172.30.200/24 network, the PCs will think that the destination exists locally and will do an ARP for it instead of sending the packet to the default gateway.

Hope that helps - pls do rate posts that help.

Paresh

[carl_townshend]

Would this be the same for any network starting 172.30 etc ? and if I had sat a 192.228/16 network and this wants to find the 192.228.1/24 netwrok, will this do the same thing ?

[josephbruce]

{Would this be the same for any network starting 172.30 etc ? and if I had sat a 192.228/16 network and this wants to find the 192.228.1/24 netwrok, will this do the same thing ?}

You can NOT (or at least SHOULD NOT) do that with the 192 network, as that is a Class C, and the 172 is a class B.

[carl_townshend]

in what networks cant I use this method, is it basically any previous network numbers can not be used i.e 192.168/16 and if i tried to reach 192.168.1/24 would it look locally for this because it woul look at the first 2 subnets and think that the rest belongs to it ?

thanks

[tdrais]

As stated above if you have your devices in a /16 network and route a /24 in the same range the devices with the /16 mask will do arps to find it rather than use the default gateway. The actually ip address don't matter only that the subnet masks are set correctly.

The reason that this works on many cisco routers is that cisco runs proxy arp by default. When a client machine issues a arp the router will respond to that arp with its mac address. It will then route the packet to the correct desination.

Just because this works this is not a good thing to do. proxy arp is one of those options that is many times turned off for security reasons