IP Addressing, Subnetting and Routing

IPv4 addresses​

An IPv4 address is 32 bits, written as four octets (192.168.10.25). It has a network part and a host part, separated by the subnet mask or CIDR prefix.

Classless addressing (CIDR) uses an explicit prefix length such as 192.168.10.0/26.

Subnetting example​

192.168.10.0/26

Prefix 26 -> mask 255.255.255.192
Host bits = 32 - 26 = 6  -> 2^6 = 64 addresses
Usable hosts = 64 - 2 = 62 (network + broadcast excluded)
Network:   192.168.10.0
Broadcast: 192.168.10.63
Range:     192.168.10.1 to 192.168.10.62

import ipaddress

net = ipaddress.ip_network("192.168.10.0/26")
print(net.netmask, net.broadcast_address, net.num_addresses - 2)
# 255.255.255.192 192.168.10.63 62

NAT​

Network Address Translation lets many private hosts share one public IP by rewriting addresses and ports at the router. It conserves IPv4 addresses but breaks true end-to-end connectivity.

IPv6​

128-bit addresses (2001:db8::1), no broadcast, built-in auto-configuration, and no NAT needed because the address space is huge.

Routing​

A router looks up the destination in its routing table using longest-prefix match and forwards to the next hop.

ICMP​

Control messages for the IP layer: ping (echo request/reply), traceroute (TTL expired), destination unreachable.

ping -c 4 example.com
traceroute example.com

The TTL field is decremented at every hop and the packet is dropped at zero, which stops routing loops.