hop 11 of 32 · 5 min

IP addresses and what a mask means

An IPv4 address is 32 bits, written as four bytes: 172.16.5.42. It is not one number. It is two, and the mask says where to cut.

The mask

172.16.5.42/24 means the first 24 bits are the network and the last 8 are the host.

code

172.16.5.42     10101100 00010000 00000101 00101010
/24 mask        11111111 11111111 11111111 00000000
                <------ network ---------> <-host->

network address 172.16.5.0
broadcast       172.16.5.255
usable hosts    172.16.5.1 to 172.16.5.254   (254 of them)

Two addresses are never assignable: all host bits zero is the network itself, all host bits one is the broadcast. That is where the minus two comes from.

Prefix	Host bits	Addresses	Usable
/24	8	256	254
/25	7	128	126
/26	6	64	62
/27	5	32	30
/30	2	4	2

Private ranges

Three blocks are reserved for internal use and never routed on the public internet:

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16

Your home router hands out addresses from one of these and translates them to its single public address on the way out. That is NAT, and it is why IPv4 has lasted long past the point it should have run out.