Unit 4 · Addressing Network Devices

4.3IPv4 addressing

An IPv4 address is 32 bits, written as four decimal octets (dotted decimal): 192.168.10.25 = 11000000.10101000.00001010.00011001. It has a network portion and a host portion, separated by the subnet mask.

19211000000255.16810101000255.1000001010255.25000110010binarymaskNetwork portion (24 bits, /24)Host portionNetwork address 192.168.10.0 · this host is number 25 on it
An IPv4 address is 32 bits in four octets; the subnet mask marks which bits are the network part and which are the host part

Key words in this topic

  • IPv4 address: 32-bit identifier written as four decimal octets.
  • Network portion / host portion: which bits identify the network, and which identify the device on that network.
  • Subnet mask / prefix length: the line between those two portions (255.255.255.0 or /24).
  • Private address: not routed on the public Internet; used inside organisations with NAT.
  • Unicast / broadcast / multicast: one, all-on-subnet, or a group.

In this section

Skills you should leave with

Given an address and mask, you should be able to say whether it is public or private, estimate its classful range for recognition, and decide whether a host should get it statically or via DHCP. The next pages (4.4–4.5) turn the mask into exact network, broadcast and usable ranges.