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Course 2: IP Subnetting and VLSMLesson 1.2 (2 of 8 in this course)7 of 98 in the CCNA series

Subnet masks and CIDR

The mask says where the network part of an address ends. Learn to read it in binary and convert it in your head.

Intermediate · 7 min read · Before this: Subnetting

After this lesson, you can convert any prefix such as /20 into a dotted mask and back, and say how many addresses and usable hosts it gives.

Subnet mask is a 32-bit value that marks which bits of an IPv4 address are the network part (1 bits) and which are the host part (0 bits). All the 1 bits come first. CIDR notation writes the same mask as a prefix length: the number of 1 bits, for example /26.

In simple terms: The mask is a ruler laid over the address. Everything under the 1s is the network's name; everything under the 0s numbers the devices inside it.

Two parts in every address

An IPv4 address is 32 bits, written as four 8-bit octets. The first part of those bits names the network; the rest numbers a device on that network. The mask is the line between them. Here is 192.168.1.100/26:

Each octet: address bits above, mask bits below.Network: 26 bitsHost: 6 bits
/26: 26 network bits (mask 1s), then 6 host bits (mask 0s). The line falls inside the 4th octet.

/26 simply counts the 1 bits in the mask. Write those 26 ones as octets and you get 255.255.255.192: three full octets of eight 1s (255 each), then 11000000, which is 192.

The only eight values a mask octet can take

Because the 1s come first, a mask octet can only be one of these values. Learn this row and you can convert any prefix:

1 bits in the octetBinaryValueBlock size (256 − value)
110000000128128
21100000019264
31110000022432
41111000024016
5111110002488
6111111002524
7111111102542
8111111112551

💡 Memory trick: each value adds the next bit: 128, +64 = 192, +32 = 224, +16 = 240, +8 = 248, +4 = 252, +2 = 254, +1 = 255. The block size column is used in the step-by-step method.

Prefix to mask

  1. Divide the prefix by 8. The whole number is how many octets are 255.
  2. The remainder is how many 1 bits go in the next octet: look it up in the table.
  3. Any octets left over are 0.
PrefixWorkingMask
/2020 = 2 × 8 + 4 → 255.255, then 4 bits = 240, then 0255.255.240.0
/2727 = 3 × 8 + 3 → 255.255.255, then 3 bits = 224255.255.255.224
/1212 = 1 × 8 + 4 → 255, then 240, then 0.0255.240.0.0
Each octet: address bits above, mask bits below.Network: 20 bitsHost: 12 bits
/20: the line falls inside the 3rd octet, so the mask there is 240 and the whole 4th octet is host bits.

Mask to prefix

Go the other way: count 8 for each 255, then add the number of 1 bits in the next octet. 255.255.255.248 is 8 + 8 + 8 + 5 = /29. 255.255.192.0 is 8 + 8 + 2 = /18.

A mask with a gap, such as 255.255.0.255 or 255.255.255.200, is not a valid subnet mask: the 1s must be continuous. (200 is 11001000, which has a 0 between 1s.)

Counting addresses and hosts

The host bits decide the size. With h host bits (32 − prefix):

  • Addresses in the subnet = 2h.
  • Usable hosts = 2h − 2. The first address is the network address and the last is the broadcast address; neither can be given to a device.
PrefixHost bitsAddressesUsable hostsTypical use
/20124,0964,094A campus building or a large site
/248256254A typical user VLAN
/2666462A department or guest Wi-Fi
/2753230Servers or printers
/30242A router-to-router link

The two exceptions. A /31 has no network or broadcast address: both of its two addresses are usable on a point-to-point link (RFC 3021). A /32 is a single address, used for loopback interfaces and host routes. The 2h − 2 rule applies from /30 downwards.

Check yourself

Predict · scenario 1

What is the dotted subnet mask for /20?

Predict · scenario 2

A router shows a mask of 255.255.255.248. What is the prefix length?

Predict · scenario 3

How many usable host addresses does a /27 subnet have?

Predict · scenario 4

Which of these is a valid subnet mask?

Practise

Masks become automatic with repetition. The subnetting practice drill asks unlimited questions and shows the worked answer; the CIDR calculator checks any conversion.

Learn more: A Step-by-Step MethodWildcard Masks