cidr 30
Connecting two routers over a /24 network would waste 252 addresses that would never belong to anyone; a /30 solves that by reserving the smallest block that still fits both ends of a point-to-point link.
This block 2 host bits only allow .1 and .2 as hosts; .0 stays reserved as the network and .3 as the broadcast, leaving no room for a third device.
203.0.113.0/24 is the TEST-NET-3 range from RFC 5737, reserved for documentation, which is why it shows up in so many carrier circuit examples without exposing a real IP.
Here the network and broadcast addresses stop being reserved; both addresses in this 2-address block (not a 4-address one) go to the two ends of the link, as RFC 3021 sets out.
Yes. It helps visualize the main subnetting outputs for both IPv4 and IPv6, including split scenarios and more detailed planning workflows.
Because a link between two routers only needs 2 usable addresses; reserving a 254-host /24 for that would waste 252 unused addresses, while a /30 delivers exactly the 4 addresses needed, 2 of them assignable.
It is a prefix RFC 3021 defined in 2000 for point-to-point links that skips the network and broadcast reservation, leaving both addresses in the 2-address block fully usable; it pays off for carriers running hundreds of links, where every saved address adds up.
Since every /30 block spans 4 addresses aligned in steps of 4, an address ending in .4 already starts a block: network at .4, usable hosts at .5 and .6, broadcast at .7, with the next block starting at .8.
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