IP Addressing & Subnetting

IP addresses identify hosts; subnetting splits a network into smaller networks using a subnet mask.

Cheat sheet

  • ▸IPv4 = 32 bits; IPv6 = 128 bits.
  • ▸Hosts per subnet = 2^(32 − prefix) − 2.
  • ▸Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.
  • ▸/24 = 255.255.255.0 (254 hosts), /26 = 255.255.255.192 (62 hosts).
  • ▸Network address: all host bits 0. Broadcast: all host bits 1.
  • ▸NAT lets many private hosts share one public IP.

CIDR quick table

PrefixMaskUsable hosts
/24255.255.255.0254
/25255.255.255.128126
/26255.255.255.19262
/27255.255.255.22430
/30255.255.255.2522

Visual

192.168.1.70/26
mask .192 → block size 64 → subnets .0 .64 .128 .192
70 is in .64 subnet: network .64, broadcast .127, hosts .65–.126

Top 10 campus interview questions

Quick quiz

1. Usable hosts in /26:

2. Which is a private IP?

3. Mask for /27:

4. IPv6 address length:

5. DHCP's first message: