Route summarization
Combine several networks into one summary route, and see whether it covers anything extra.
One CIDR block per line. 4 networks.
What the summary covers
172.16.0.0/22 drawn to scale with your networks inside it
- 172.16.0.0/24
- 172.16.1.0/24
- 172.16.2.0/24
- 172.16.3.0/24
How summarization works
A summary route (or supernet) is one shorter prefix that covers several networks, so routers advertise one line instead of many.
Write the networks in binary and find the common leading bits, the bits that are the same in every network, counting from the left. That count is the summary's prefix length. For 172.16.0.0 to 172.16.3.0 the third octets are 00000000, 00000001, 00000010, 00000011: the first 6 bits match, so 16 + 6 = /22.
Why exact matters: a summary that covers addresses you do not have still pulls traffic for them toward you. Your router then has no specific route and drops it, which is called black-holing. A clean summary needs a power-of-two number of equal neighbors starting on a matching boundary (for example 4 × /24 starting at a multiple of 4).