Subnetting Cheat Sheet — CIDR, Subnet Mask and Host Count Chart

Everything you need on one page: CIDR prefix, subnet mask, block size, usable hosts and wildcard mask. If you are learning the method rather than looking up a value, start with subnetting explained.

Table of Contents

The Four Things Worth Memorising

Memorise	Value
Bit values in an octet	128, 64, 32, 16, 8, 4, 2, 1
Valid mask octets	0, 128, 192, 224, 240, 248, 252, 254, 255
Block size (magic number)	256 − mask octet
Usable hosts	2^{host bits} − 2

A mask octet that is not in that second row is invalid. Mask bits are contiguous from the left, so 100, 200 and 255.255.255.100 can never be correct.

Full CIDR Chart — /8 to /32

CIDR	Subnet mask	Wildcard mask	Block size	Total addresses	Usable hosts
/8	255.0.0.0	0.255.255.255	—	16,777,216	16,777,214
/9	255.128.0.0	0.127.255.255	128 (2nd)	8,388,608	8,388,606
/10	255.192.0.0	0.63.255.255	64 (2nd)	4,194,304	4,194,302
/11	255.224.0.0	0.31.255.255	32 (2nd)	2,097,152	2,097,150
/12	255.240.0.0	0.15.255.255	16 (2nd)	1,048,576	1,048,574
/13	255.248.0.0	0.7.255.255	8 (2nd)	524,288	524,286
/14	255.252.0.0	0.3.255.255	4 (2nd)	262,144	262,142
/15	255.254.0.0	0.1.255.255	2 (2nd)	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	128 (3rd)	32,768	32,766
/18	255.255.192.0	0.0.63.255	64 (3rd)	16,384	16,382
/19	255.255.224.0	0.0.31.255	32 (3rd)	8,192	8,190
/20	255.255.240.0	0.0.15.255	16 (3rd)	4,096	4,094
/21	255.255.248.0	0.0.7.255	8 (3rd)	2,048	2,046
/22	255.255.252.0	0.0.3.255	4 (3rd)	1,024	1,022
/23	255.255.254.0	0.0.1.255	2 (3rd)	512	510
/24	255.255.255.0	0.0.0.255	—	256	254
/25	255.255.255.128	0.0.0.127	128	128	126
/26	255.255.255.192	0.0.0.63	64	64	62
/27	255.255.255.224	0.0.0.31	32	32	30
/28	255.255.255.240	0.0.0.15	16	16	14
/29	255.255.255.248	0.0.0.7	8	8	6
/30	255.255.255.252	0.0.0.3	4	4	2
/31	255.255.255.254	0.0.0.1	2	2	2 (RFC 3021)
/32	255.255.255.255	0.0.0.0	1	1	1

Bolded rows are the ones that appear most often in exams and in real designs.

Binary Reference

Decimal	Binary	Bits set
0	00000000	0
128	10000000	1
192	11000000	2
224	11100000	3
240	11110000	4
248	11111000	5
252	11111100	6
254	11111110	7
255	11111111	8

Converting a full address is covered in decimal to binary conversion.

Wildcard Masks

A wildcard mask is the inverse of a subnet mask — subtract each octet from 255. Used by ACLs and OSPF network statements, where 0 means “must match” and 1 means “ignore”.

Subnet mask:   255.255.255.192
               255-255 . 255-255 . 255-255 . 255-192
Wildcard:        0     .   0     .   0     .   63

access-list 101 permit ip 192.168.1.0 0.0.0.63 any
router ospf 1
 network 192.168.1.0 0.0.0.63 area 0

Private Address Ranges

Range	CIDR	Addresses	Typically used by
10.0.0.0 – 10.255.255.255	10.0.0.0/8	16,777,216	Enterprises, cloud VPCs
172.16.0.0 – 172.31.255.255	172.16.0.0/12	1,048,576	Medium networks, Docker
192.168.0.0 – 192.168.255.255	192.168.0.0/16	65,536	Home and small office

The 172 range is 172.16 to 172.31 only — not all of 172. Full detail in IPv4 addressing.

Special Addresses

Range	Meaning
127.0.0.0/8	Loopback
169.254.0.0/16	APIPA — DHCP failed
100.64.0.0/10	Carrier-grade NAT
224.0.0.0/4	Multicast
255.255.255.255	Limited broadcast
0.0.0.0/0	Default route

Fast Lookup — Hosts Needed to Prefix

You need	Use	You get
1–2 hosts	/30	2
3–6	/29	6
7–14	/28	14
15–30	/27	30
31–62	/26	62
63–126	/25	126
127–254	/24	254
255–510	/23	510
511–1022	/22	1,022

Round up — always. Needing 65 hosts means a /25, not a /26.

Finding a Subnet in Three Steps

Given: 192.168.5.100/27

1. Block size  = 256 - 224 = 32
2. Blocks      = 0, 32, 64, 96, 128, 160, 192, 224
   100 falls in the 96 block
3. Network     = 192.168.5.96
   Broadcast   = 96 + 32 - 1 = 192.168.5.127
   Usable      = 192.168.5.97 - 192.168.5.126

Check any answer instantly with the subnet calculator, then test yourself against the practice questions. For unequal subnet sizes in one network, see VLSM.

Frequently Asked Questions

What is the block size in subnetting?

256 minus the interesting mask octet. It is the increment between consecutive subnet addresses — for a /28 (mask 240) the block size is 16, so subnets start at 0, 16, 32, 48 and so on.

How do I convert a subnet mask to CIDR?

Count the 1 bits. 255.255.255.192 is 8 + 8 + 8 + 2 = /26. The bits-set column in the binary table above gives the count for each octet value.

What is a wildcard mask used for?

ACLs and OSPF network statements. It is the inverse of the subnet mask — subtract each octet from 255. A /26 mask of 255.255.255.192 becomes wildcard 0.0.0.63.

Why does a /30 give only 2 usable hosts?

It has 4 total addresses, and two are always consumed by the network and broadcast addresses. That leaves exactly 2, which is why /30 is the standard for point-to-point WAN links.

Can I use a /31?

Yes, on point-to-point links. RFC 3021 removes the network and broadcast addresses for that case, giving 2 usable addresses from 2 total. Most modern router platforms support it.

What is the largest private network I can build?

10.0.0.0/8 gives 16,777,214 usable addresses — the largest of the three RFC 1918 ranges.

Written by

Gurpreet Singh

Hey! I"m Gurpreet Singh and I Have 7+ Years of experience in the Network & Security Domain as well as the Cloud Infra Domain. I am Certified with Cisco ( CCNA ), CheckPoint ( CCSA ), 1xAWS, 3xAZURE, and 3xNSE. So I love to share my tech knowledge with you.