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” (10.0.0.1) 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 255.255.255.0. Having said that, rarely some devices (Cisco routers in the dustier parts of their code) require the reverse – 0.0.0.255.
An alternative approach is to use the CIDR format to specify both the IP address of the device and the size of the network – 10.2.9.21/24. 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 10.2.9.21/24 Address: 10.2.9.21 00001010.00000010.00001001. 00010101 Netmask: 255.255.255.0 = 24 11111111.11111111.11111111. 00000000 Wildcard: 0.0.0.255 00000000.00000000.00000000. 11111111 => Network: 10.2.9.0/24 00001010.00000010.00001001. 00000000 HostMin: 10.2.9.1 00001010.00000010.00001001. 00000001 HostMax: 10.2.9.254 00001010.00000010.00001001. 11111110 Broadcast: 10.2.9.255 00001010.00000010.00001001. 11111111 Hosts/Net: 254 Class A, Private Internet