Subnet Cheat Sheet

Subnet cheat sheet

The table every network admin eventually memorises, plus the reserved ranges and the IPv6 side.

Fits on one A4 / Letter page in portrait; use “Save as PDF” in the print dialog to keep a copy.

IPv4 prefix table

/	Mask	Wildcard	Addresses	Usable
0	0.0.0.0	255.255.255.255	4,294,967,296	4,294,967,294
1	128.0.0.0	127.255.255.255	2,147,483,648	2,147,483,646
2	192.0.0.0	63.255.255.255	1,073,741,824	1,073,741,822
3	224.0.0.0	31.255.255.255	536,870,912	536,870,910
4	240.0.0.0	15.255.255.255	268,435,456	268,435,454
5	248.0.0.0	7.255.255.255	134,217,728	134,217,726
6	252.0.0.0	3.255.255.255	67,108,864	67,108,862
7	254.0.0.0	1.255.255.255	33,554,432	33,554,430
8	255.0.0.0	0.255.255.255	16,777,216	16,777,214
9	255.128.0.0	0.127.255.255	8,388,608	8,388,606
10	255.192.0.0	0.63.255.255	4,194,304	4,194,302
11	255.224.0.0	0.31.255.255	2,097,152	2,097,150
12	255.240.0.0	0.15.255.255	1,048,576	1,048,574
13	255.248.0.0	0.7.255.255	524,288	524,286
14	255.252.0.0	0.3.255.255	262,144	262,142
15	255.254.0.0	0.1.255.255	131,072	131,070
16	255.255.0.0	0.0.255.255	65,536	65,534
17	255.255.128.0	0.0.127.255	32,768	32,766
18	255.255.192.0	0.0.63.255	16,384	16,382
19	255.255.224.0	0.0.31.255	8,192	8,190
20	255.255.240.0	0.0.15.255	4,096	4,094
21	255.255.248.0	0.0.7.255	2,048	2,046
22	255.255.252.0	0.0.3.255	1,024	1,022
23	255.255.254.0	0.0.1.255	512	510
24	255.255.255.0	0.0.0.255	256	254
25	255.255.255.128	0.0.0.127	128	126
26	255.255.255.192	0.0.0.63	64	62
27	255.255.255.224	0.0.0.31	32	30
28	255.255.255.240	0.0.0.15	16	14
29	255.255.255.248	0.0.0.7	8	6
30	255.255.255.252	0.0.0.3	4	2
31	255.255.255.254	0.0.0.1	2	2
32	255.255.255.255	0.0.0.0	1	1

10.0.0.0/8	private (RFC 1918)
172.16.0.0/12	private: 172.16.0.0 – 172.31.255.255
192.168.0.0/16	private
100.64.0.0/10	carrier-grade NAT (RFC 6598)
127.0.0.0/8	loopback
169.254.0.0/16	link-local / APIPA (no DHCP)
192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24	documentation (TEST-NET-1/2/3)
224.0.0.0/4	multicast
240.0.0.0/4	reserved
255.255.255.255	limited broadcast
0.0.0.0/8	“this network”; 0.0.0.0 = any address when binding

Block size = 256 − the interesting mask octet (mask .192 → 64).
Network = the address rounded down to a multiple of the block size in that octet.
Broadcast = network + block size − 1; hosts are between.
Usable = 2^{host bits} − 2 (except /31 and /32).

Example: 172.16.5.77/26 → mask 255.255.255.192, block 64 → network 172.16.5.64, broadcast .127, hosts .65–.126 (62).

/32	typical ISP allocation
/48	one site (65,536 /64s)
/56	home customer (256 /64s)
/64	one LAN; never smaller for SLAAC
/127	point-to-point link
/128	single host

2000::/3	global unicast (2xxx:, 3xxx:)
fc00::/7 (fd…)	unique local (private)
fe80::/10	link-local (every interface has one)
ff00::/8	multicast; ff02::1 all nodes, ff02::2 all routers
::1	loopback
::	unspecified / any
::ffff:a.b.c.d	IPv4-mapped
2001:db8::/32	documentation
64:ff9b::/96	NAT64

8 groups of 4 hex digits; drop leading zeros; one :: for the longest zero run.
No broadcast, no network/broadcast addresses lost: every address in a /64 is usable.
In URLs: http://[2001:db8::1]:8080/.
Reverse DNS: nibbles reversed under ip6.arpa.

Which tool computes this for a specific address?

The subnet calculator (IPv4) and IPv6 calculator give the network, range, mask in binary and splitting for any address you enter.