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There are plenty of these netmask references out there, but I prefer my own: hence this Tech Tip. We've never seen anybody use a network larger than a /4 (256M hosts), so we've truncated the table at that point.

Netmask Quick Reference
# bits # hosts Usable
hosts 		netmask Cisco mask
/4 268435456 268435454 240.0.0.0 15.255.255.255
/5 134217728 134217726 248.0.0.0 7.255.255.255
/6 67108864 67108862 252.0.0.0 3.255.255.255
/7 33554432 33554430 254.0.0.0 1.255.255.255
/8 16777216 16777214 255.0.0.0 class A network 0.255.255.255
/9 8388608 8388606 255.128.0.0 0.127.255.255
/10 4194304 4194302 255.192.0.0 0.63.255.255
/11 2097152 2097150 255.224.0.0 0.31.255.255
/12 1048576 1048574 255.240.0.0 0.15.255.255
/13 524288 524286 255.248.0.0 0.7.255.255
/14 262144 262142 255.252.0.0 0.3.255.255
/15 131072 131070 255.254.0.0 0.1.255.255
/16 65536 65534 255.255.0.0 class B network 0.0.255.255
/17 32768 32766 255.255.128.0 0.0.127.255
/18 16384 16382 255.255.192.0 0.0.63.255
/19 8192 8190 255.255.224.0 0.0.31.255
/20 4096 4094 255.255.240.0 0.0.15.255
/21 2048 2046 255.255.248.0 0.0.7.255
/22 1024 1022 255.255.252.0 0.0.3.255
/23 512 510 255.255.254.0 0.0.1.255
/24 256 254 255.255.255.0 class C network 0.0.0.255
/25 128 126 255.255.255.128 0.0.0.127
/26 64 62 255.255.255.192 0.0.0.63
/27 32 30 255.255.255.224 0.0.0.31
/28 16 14 255.255.255.240 0.0.0.15
/29 8 6 255.255.255.248 0.0.0.7
/30 4 2 255.255.255.252 0.0.0.3
/31   point to point links only
/32 1 1 255.255.255.255 single IP address use host notation

What's a netmask?

All devices on a local network have a unique IP address, but each address is inherently divided into two parts, a shared network part, and a unique host part, and this information is used by the TCP/IP stack for routing. When sending traffic to a machine with a different network part, it must be sent through a router for final delivery.

The dividing line between the network and host parts is determined by the subnet mask, and it's often seen in 255.255.255.0 notation. It looks like an IP address, and it uses a "1" bit to select, or "mask" the network part.

[IP address with mask]

In this case, the netmask of 255.255.255.248 represents 29 bits of network and 3 bits of host (totalling 32 bits, of course), and this give 8 possible IP addresses in this range. The first and last of the range are reserved addresses, giving 6 usable addresses that may be assigned to a device.

216.39.106.160 zero broadcast
216.39.106.161 available
216.39.106.162 available
216.39.106.163 available
216.39.106.164 available
216.39.106.165 available
216.39.106.166 available
216.39.106.167 ones broadcast

What's that Cisco notation?

When creating ACLs for Cisco routers, one specifies networks using a base IP address and what looks like an inverted netmask: rather than set a one bit for each part of the address that's a network, they set a one bit for each part that's a host.

Many consider this quite annoying, as it's one more thing to have to remember.

First published: 2005/07/17