a strange phenomena

[cobra_he]

The FastEternet 0 interface of a Cisco 1750 Router is attached to a PC with a cross-over UTP5.The IP address of F0 and PC is 210.15.86.31/30,210.15.86.30/30.The status of F0 is line proctocol up.But,they can't ping each other successfully.With debug command,i find router can receive echo packet from the PC,but no echo-reply.Afterward,i change their IP address to 192.168.1.1/24 and 192.168.1.2/24,PC and router can ping each other successfully.Can you tell me why?

[rwiesmann]

Hi

You need to configure the right ip address on the interface...the 210.15.86.30 / 30 is o.k but the 210.15.86.31 /31 is the broadcast address of this subnet.

So use:

Network: 210.15.86.28 /30

PC: 210.15.86.29 /30

F0 Router: 210.15.86.30 /30

Broadcast: 210.15.86.31 /30

I guess that was you problem.

Roger

[a.manosca · Accepted solution]

Probably because 210.15.86.31/30 is a broadcast address on the subnet

210.15.86.28/30

210.15.86.28 ---> network

210.15.86.29 ---> host

210.15.86.30 ---> host

210.15.86.31 ---> broadcast

HTH.