Feb 242019

Normally when you set an IP address manually on an interface you do not get a whole lot of choice of how it is done – very often you have to specify the IP address itself and a network mask. The addresses and masks are almost always specified as “dotted quads” ( rather than the real address in binary or decimal (167772161).

The network mask specifies what parts of the IP address are the network address and which are the host address – to determine whether a destination needs to go via a gateway or is on the local network. This is expressed as a bitmask like Having said that, rarely some devices (Cisco routers in the dustier parts of their code) require the reverse –

An alternative approach is to use the CIDR format to specify both the IP address of the device and the size of the network – This is used (at least) on Palo Alto Networks firewalls and is probably the simplest way of configuring a network address I have come across.

Having configured hundreds of devices with static addresses … and helped solve oodles of network configuration issues, I feel that the CIDR format method is likely to be far less error prone.

If you do need to set a netmask, use ipcalc to check what it is (and use it to cut&paste rather than risk typos) :-

✓ mike@pica» ipcalc 
Address:            00001010.00000010.00001001. 00010101
Netmask: = 24   11111111.11111111.11111111. 00000000
Wildcard:            00000000.00000000.00000000. 11111111
Network:          00001010.00000010.00001001. 00000000
HostMin:             00001010.00000010.00001001. 00000001
HostMax:           00001010.00000010.00001001. 11111110
Broadcast:           00001010.00000010.00001001. 11111111
Hosts/Net: 254                   Class A, Private Internet
Through The Gateway