Route Summarization Calculator

About this interactive

Route summarization replaces a group of networks with one route that covers them all, so a router advertises one line instead of many. The summary is the longest prefix the networks have in common. Write the octet where they start to differ in binary, keep the leading bits that match across all of them, and zero the rest. 172.18.4.0/24 to 172.18.7.0/24 differ in the third octet: 4 is 00000100 and 7 is 00000111, and the first six bits match, so the summary keeps 16 + 6 = 22 bits: 172.18.4.0/22. When the summary is shorter than the class default, as 200.30.6.0/23 is for two Class C networks, it is called a supernet. A summary always covers a power of two of addresses, so it can take in networks that are not there. Summarizing 172.18.4.0/24 to 172.18.6.0/24 still needs the /22, and that route also covers 172.18.7.0/24. If 172.18.7.0/24 is in use behind some other router, this one now attracts traffic meant for it. That is the discontiguous-network problem, and it is why you check what a summary covers before you advertise it. Type addresses in dotted decimal and prefix lengths as a number. Anything you miss shows the answer and the binary that gives it. Seven of the twenty-three questions are drawn each run.

What you'll learn

Calculate the summary route for a group of contiguous networks by finding the bits they share, including supernets beyond the classful boundary
Identify the networks a summary route covers that are not behind the router, the risk with discontiguous networks

Aligned to

CompTIA Network+

1.7 Given a scenario, use appropriate IPv4 network addressing.

Cisco CCNA

1.6 Configure and verify IPv4 addressing and subnetting

Key terms

Route Summarization: The process of combining multiple contiguous network routes into a single advertised route to reduce routing table size and simplify routing updates.
Summary Route: A single static route that represents a range of network addresses, reducing the number of entries in a routing table.
Supernetting: A form of route summarization where multiple networks are grouped into a single route that crosses classful network boundaries, resulting in a prefix shorter than the default class length.
Discontiguous Networks: Networks that share the same summary address range but are not physically located behind the same router, creating routing ambiguity when a summary route is advertised.
Prefix Length: The number of consecutive 1-bits in a subnet mask, written after a slash (e.g., /24), that defines how many bits of an IP address represent the network portion versus the host portion. A /24 prefix means the first 24 bits identify the network, leaving 8 bits for host addresses.

Topics

Route Summarization Supernetting Summary Route Subnetting Interactive Calculation

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