Networking
Guess first
How many hosts fit in a /30?
Most students get this wrong: a /30 subnet. Guess before I explain.
Quick check · tap an answer to continue
Lesson notes
Key points
- A /30 leaves two host bits, so the block holds four addresses, but the first is the network ID and the last is broadcast.
- That leaves exactly two usable hosts, which is why /30 is the classic choice for a point-to-point link between two routers.
Takeaway: A /30 has 4 addresses but only 2 usable hosts.
Quick check: How many usable host addresses does a /30 subnet give you?
- 4
- 2
- 30
Usable hosts = 2^(host bits) − 2; a /30 gives 2, perfect for router links.
2² − 2Point-to-point
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