Supernetting — Combining Blocks

Kite Studio's second office has grown into four networks: 192.168.16.0/24, 192.168.17.0/24, 192.168.18.0/24 and 192.168.19.0/24 (head office keeps 192.168.10.0/24). The head-office router could hold four separate routes to reach them, one per network. Or it could hold just one. Supernetting runs subnetting in reverse: it merges neighbouring blocks into one bigger prefix, so routers carry shorter tables and fewer updates.

The rule

Keep the bits they share; the shared bits are the new prefix.

A supernet (also called a summary or aggregate route) is a single prefix that covers several smaller networks exactly. You find it by writing the networks in binary, lining them up, and keeping only the leading bits they all have in common. The count of those shared bits is the new prefix length.

A clean supernet needs three things:

• the networks are contiguous (no gaps between them);
• their number is a power of two (2, 4, 8, …);
• the first network is a multiple of that number in the octet that changes, so the group sits on a proper boundary.

Why it matters: fewer routes to store and look up, and a change deep inside one office does not ripple across every router that only knows the summary. ISPs aggregate the blocks of their customers in exactly this way before announcing them to the rest of the internet.

The shortcut

Only one octet usually changes, so convert just that octet to binary and count how many leading bits match. Add that count to the octets that were fully shared (8 per octet).

The trap is a summary that claims more than you have. If the office only had networks 16, 17 and 18, a /22 would also attract traffic for 192.168.19.0, which does not exist there, and that traffic would be dropped. A summary must match what really sits behind it.

Drills

Answer fast — trust the rule.

How many /24 networks fit inside one /22:

Remember

• Keep the bits they share; the shared bits are the new prefix.
• Clean supernets are contiguous, a power of two in number, and start on a matching boundary.
• Never summarise more than you actually have behind the router.