IPv4 Subnetting Guide

The bit model

An IPv4 address is 32 bits. A prefix /p reserves the first p bits for network membership and leaves 32−p host bits. The subnet mask has p contiguous one-bits. Network calculation is a bitwise AND; the broadcast fills the remaining bits with ones. An address entered as 192.168.50.37/26 belongs to 192.168.50.0/26, not a subnet starting at .37.

Capacity and boundary reasoning

A /26 has 64 total addresses. Ordinary multi-access IPv4 subnets reserve the network and broadcast endpoints, giving 62 usable hosts. /31 point-to-point addressing can use both addresses under RFC 3021; /32 identifies one address/host route. Do not apply the two-reserved-address rule blindly to those special prefixes. Block boundaries must align to the prefix.

Designing a VLSM plan

Start from a parent prefix and count present endpoints, gateways and realistic growth. Sort requirements largest first and round each hosts+2 count up to a power of two. Allocate aligned blocks, recording network, range, broadcast and mask. Verify all children remain inside the parent and never overlap. Address capacity alone does not prove a routing or VLAN design is valid.

Operational verification

Check the interface address/mask, compare network membership and verify that the default gateway is on the intended local link. Duplicate addresses can mimic intermittent failures. A reachable local peer with unreachable remote services points toward gateway, routes or policy rather than subnet arithmetic alone.

Signal / protocol flow

1. 132-bit address
2. 2AND contiguous mask
3. 3Network boundary
4. 4Usable range / broadcast

Technical terminology

• CIDR: classless prefix notation
• Subnet mask: network-bit selector
• Wildcard: inverse of mask
• VLSM: differently sized child subnets

Worked example

192.168.50.37/26 → network .0, broadcast .63, usable .1–.62, mask 255.255.255.192, wildcard 0.0.0.63. For 50 and 20 hosts inside /24 allocate /26 first, then /27 on the next aligned boundary.

Configuration reference

# Reference addressing plan (not a command executed here)
LAN-A: 192.168.50.0/26
Gateway: 192.168.50.1
Client: 192.168.50.10/26
LAN-B: 192.168.50.64/27

Related practice

• IP Subnet Calculator
• VLSM Planner
• Subnetting Quick Sheet