Subnets and gateways
Work out subnet sizes, ranges and gateways quickly and check an addressing plan for mistakes.
- Level
- Intermediate
- Length
- About 50 minutes
- Contents
- 3 lessons · 1 video · final exam
- Status
- Published · updated 1 Oct 2026
Skills you'll practise
- Calculate the number of usable hosts for a prefix length
- Find the network, broadcast and usable range for an address
- Choose the smallest prefix that fits a requirement
- Spot overlapping subnets and misplaced gateways in an addressing plan
Course outline
- 1.Subnets and gateways without the headacheVideo · 2 min
- 2.The block-size methodLesson · 18 min
- 3.Subnetting beyond the last octetLesson · 18 min
- 4.Masks, prefixes and checking someone else's planLesson · 15 min
- 5.Subnets and gateways: knowledge checkKnowledge check · 19 questions
- 6.Subnets and gateways: practical exerciseKnowledge check · 1 question
- 7.Final exam7 questions · passing it completes the course, so people who already know the material can test out
Sources it draws on
The lessons and questions are written from these references, so learners can go back to the original.
- RFC 4632: Classless Inter-domain Routing (CIDR)
- RFC 1918: Address Allocation for Private Internets
- CIDR subnetting method: block size = 256 − mask octet
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