Adjusting your netmask to correct routing problems

If you are unable to communicate with networks that your gateways can reach, you may need to adjust your netmask. This may be the case if netstat -r on a host displays your local network address incorrectly. For example, it displays a class B two-octet network address, such as 135.7, when you know that class C subnetting should be in operation and its network address should be displayed as 135.7.4. An incorrect netmask will result in a host having incorrect entries in its routing table, and will prevent it from receiving routing updates broadcast from local routers.

To adjust the netmask on a host:

1. Run the Network Configuration Manager.

2. For each network adapter, check whether the correct TCP/IP configuration values have been defined for its netmask and broadcast address. If necessary, change the values in these fields.

   For example, a network adapter configured with IP address ``172.16.4.25'' might be shown as using the class B netmask ``255.255.0.0'' and broadcast address ``172.16.255.255''. However, the local network is actually a class C subnet of a class B network, and so it should be configured with the class C netmask ``255.255.255.0'' and broadcast address ``172.16.4.255''.

3. After making the necessary changes, exit the Network Configuration Manager, and then shut down and reboot the system.