IPv4 addressing

Every device on an IP network needs an address — but an IPv4 address isn't just an ID. It carries two pieces of information at once: which network the device is on, and which host it is on that network. Routers only care about the first part.

32 bits, four octets

An IPv4 address is 32 bits long. To make it readable, we split it into four 8-bit octets and write each one in decimal, separated by dots.

The prefix length (/24) says how many bits, counted from the left, belong to the network. The remaining bits identify the host.

• With /24, the first three octets are the network: 192.168.10.
• The last octet, 25, is this host.
• Every device on this LAN shares 192.168.10 — that's how they know they can talk directly.

Network, broadcast and usable hosts

Two addresses in every subnet are reserved:

• the network address — all host bits set to 0 (192.168.10.0)
• the broadcast address — all host bits set to 1 (192.168.10.255)

Everything in between can be assigned to devices. With 8 host bits, that's 2⁸ − 2 = 254 usable hosts.

The prefix doesn't have to fall on an octet boundary. Here's a /26: two more bits move to the network side, and the subnet shrinks.

Why routers care

When a packet leaves a host, each router compares the destination's network bits with its routing table and forwards the packet one hop closer. The host bits only matter on the final network.

Configure it on a Cisco router

R1> enableR1# configure terminalR1(config)# interface g0/0R1(config-if)# ip address 192.168.10.1 255.255.255.0R1(config-if)# no shutdownR1(config-if)# endR1# show ip interface briefInterface              IP-Address      OK? Method Status    ProtocolGigabitEthernet0/0     192.168.10.1    YES manual up        up

In the lab, you'll address a small topology in Packet Tracer and verify connectivity with ping.