CAB222 - Lecture 4 IPv4 subnetting and super-netting

Subnetting

Splitting one address range into multiple smaller networks (subnets) so each subnet is its own broadcast domain.

Why subnet

Reduce broadcasts, organise by department/visitor, support different media, and connect separate sites under one network ID.

Subnet mask

32-bit mask that extends the network portion into the host field to create subnets (network/subnet/host).

Borrowing bits (subnetting)

Take n host bits to create 2^n subnets, leaving enough host bits for devices per subnet.

Hosts per subnet

With h host bits, usable hosts = 2^h − 2 (network ID and broadcast reserved).

Calculation recipe

Pick required subnets → choose n so 2^n ≥ needed → borrow n highest host bits → compute new mask → verify usable hosts meet needs.

Worked example (/26)

Class C split into 4 subnets by borrowing 2 bits → /26 (255.255.255.192) → four /26s with 62 usable hosts each.

Subnetting rules

All-zeros host bits = subnet (network) address; all-ones host bits = subnet broadcast address.

External view

Outside routers see only the parent network; internal subnet/host details remain hidden.

VLSM (Variable Length Subnet Masking)

Use different masks within one block to fit mixed host counts (e.g., one /26 and two /27s).

Supernetting (route summarisation)

Combine multiple contiguous networks into a larger block to shrink routing tables.

Borrowing bits (supernetting)

Move the mask toward fewer bits (aggregate adjacent networks) so all share the same prefix.

Benefits of supernetting

Fewer routes, less CPU/memory/bandwidth for routing, and improved stability.

CIDR (Classless Inter-Domain Routing)

Allows any prefix length (/23, /26) for efficient allocation; enables subnetting and supernetting.

CIDR examples

Subnet a /24 into four /26s; aggregate four adjacent /24s into one /22 for advertisement.