If a network interface was configured with the wrong subnet mask as can happen when the default subnet is selected with a variable length subnet mask, a simple configuration change will fix it.
Consider a host that is assigned the IP address 10.50.90.15 in the class C subnet 10.50.90.0/24. The normal subnet mask for a class A 10.* subnet is 255.0.0.0, and this is the value that an operating system will guess given that IP address information alone. To correct this problem permanently so that it will persist after the host reboots, edit the /etc/netmask file and add the following line:
10.50.90.0 255.255.255.0
To reconfigure the interface, say hme0, immediately without rebooting the system, run the following as root:
ifconfig hme0 10.50.90.15 netmask 255.255.255.0
