Why ARP failed when the netmask of the router does not match?

johnifanx98

host
======
192.168.10.11
255.255.255.0

router
======
192.168.10.1
255.255.255.128


I run it in PT. The host ARPed for the router's IP, 192.168.10.1, but no ARP response came back from the router.

I understood the netmask of the router does not match with the host. However, should it be the reason of blocking ARP?

ARP is broadcast on a layer 2 broadcast address, and the router should respond because the router has no way to detect the netmask mismatch. Where am I wrong?

networker050184

That should work as far as I know. I'd venture a guess that this is a limitation with packet tracer. I wouldn't try tetsting obscure things such as this on a simulator.

boredgamelad

This works for me in PT, both with the router directly connected and with a switch between them:

Router#show int fa0/0
FastEthernet0/0 is up, line protocol is up (connected)
  Hardware is Lance, address is 000c.cfbd.c501 (bia 000c.cfbd.c501)
  Internet address is 192.168.10.1/25

Router#show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  192.168.10.1            -   000C.CFBD.C501  ARPA   FastEthernet0/0
Internet  192.168.10.11           0   0005.5EE6.BAC8  ARPA   FastEthernet0/0

PC>ipconfig

IP Address......................: 192.168.10.11
Subnet Mask.....................: 255.255.255.0
Default Gateway.................: 0.0.0.0

PC>arp -a
  Internet Address      Physical Address      Type
  192.168.10.1          000c.cfbd.c501        dynamic

I did notice that the router doesn't always respond to the arp request immediately. This may be one of those little niggling issues that PT suffers from.

Mrock4

Worked for me first time in GNS3.