Netmask Quick Reference Chart

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	31	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.

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