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Core concepts

• CIDR notation expresses network/prefix length (e.g., /24). Borrow bits from host portion to create smaller subnets.
• Borrow bits from host portion to create smaller subnets.
• Network = base address AND mask; broadcast = base OR inverted mask.

Example: split a /24 into four /26s

• Base block: 192.168.10.0/24
• New mask: /26 -> 255.255.255.192
• Subnets:
  • 192.168.10.0/26 (hosts .1–.62)
  • 192.168.10.64/26 (hosts .65–.126)
  • 192.168.10.128/26 (hosts .129–.190)
  • 192.168.10.192/26 (hosts .193–.254)

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1. Determine block size: 256 - last_octet_of_mask (for IPv4).
2. Walk the block size to enumerate ranges.
3. Keep network and broadcast reserved; others are usable.
4. Determine block size: 256 - last_octet_of_mask (for IPv4).
5. Walk the block size to enumerate ranges.
6. Keep network and broadcast reserved; others are usable.
7. Determine block size: 256 - last_octet_of_mask (for IPv4).
8. Walk the block size to enumerate ranges.
9. Keep network and broadcast reserved; others are usable.
10. Determine block size: 256 - last_octet_of_mask (for IPv4).
11. Walk the block size to enumerate ranges.
12. Keep network and broadcast reserved; others are usable.
13. Determine block size: 256 - last_octet_of_mask (for IPv4).
14. Walk the block size to enumerate ranges.
15. Keep network and broadcast reserved; others are usable.
16. Determine block size: 256 - last_octet_of_mask (for IPv4).
17. Walk the block size to enumerate ranges.
18. Keep network and broadcast reserved; others are usable.

IPv6 reminder

• No broadcasts; use solicited-node multicast.
• Subnet on nibble boundaries when possible (e.g., /56 -> /60 -> /64).

Practice drill

• Carve 10.20.0.0/22 into /24s. Answer: .0/24, .1/24, .2/24, .3/24.
• Carve 172.16.32.0/20 into /22s. Answer increments by 4 in the third octet.