Host and Router Subnet Mask Mismatch Example

In classful IP networks, every router and host in the same major network should share a common subnet mask. If there are disagreements on the length of the subnet mask, packets are not routed correctly.

Table 7-10 shows how a UNIX host and a router will interpret an IP address differently if they have different subnet masks specified for the same major network.

Table 7-10 Host and Router Subnet Mask Mismatch Example

Routing Information

Host Value

Router Value

Destination IP address

192.31.7.49

192.31.7.49

Subnet mask

255.255.255.240

255.255.255.224

Interpreted address

Subnet address 48, host 1

Subnet address 32, host 17

The host interprets the IP address 192.31.7.49 as being Host 1 on the third subnet (subnet address 48). However, because it is using a different subnet mask, the router interprets the address as being Host 17 on the first subnet (subnet address 32). Depending on the network topology and the router configuration, packets destined for IP address 192.31.7.49 might be sent to the wrong destination host, sent from the wrong interface, or dropped altogether.

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