Memory hook: Prefix is the boundary; longest match wins.
Must remember
IPv4 has 32 bits. CIDR /n reserves n network bits, leaving 32-n host bits. A conventional IPv4 subnet has 2^(32-n) addresses and usually two reserved endpoints. /31 point-to-point and /32 host routes are exceptions to the usual minus-two rule.
Worked example: 192.168.10.77/26 has blocks of 64: network .64, broadcast .127, ordinary hosts .65–.126, 62 usable. Four /26 networks fit within a /24. For VLSM, allocate the largest required blocks first and check overlap.
| Range | Meaning |
|---|---|
| 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 | RFC1918 private addresses; not public Internet destinations. |
| 169.254.0.0/16 | IPv4 link-local/APIPA; often a DHCP failure clue. |
| 127.0.0.0/8 | IPv4 loopback. |
| 224.0.0.0/4 | IPv4 multicast. |
| ::1 / :: | IPv6 loopback / unspecified address. |
| fe80::/10 | IPv6 link-local. |
| fc00::/7 | IPv6 unique-local range. |
| ff00::/8 | IPv6 multicast. |
IPv6 uses 128 bits in hexadecimal groups. Remove leading zeros and compress one consecutive run of zero groups with ::; using it twice would make the address ambiguous. Many LAN subnets use /64, but do not assume every IPv6 prefix is /64.
ARP discovers IPv4 next-hop MAC addresses on a local link. IPv6 Neighbor Discovery uses ICMPv6; blocking all ICMPv6 can break normal operation. Router advertisements supply on-link/default-router information and can support SLAAC. DHCPv6 does not replace every router-advertisement function.
Historical class A/B/C boundaries are vocabulary; current route decisions use actual prefixes. Two hosts with mismatched masks can disagree about whether traffic is local. Verify address, prefix, gateway and duplicate-address evidence together.
Choose under exam pressure
| Requirement | Choice and reason |
|---|---|
| Need 50 ordinary IPv4 hosts | A /26 provides 62 usable addresses. |
| Host has 169.254 address unexpectedly | Investigate DHCP/link configuration. |
| IPv6 address works locally but not remotely | Check router advertisements/default route and policy. |
Traps
- 172.16/12 covers 172.16 through 172.31, not all 172 addresses.
- A default gateway must be reachable through the host’s local routing configuration.
Active recall
1. How many total addresses in /27?
32; ordinarily 30 host addresses.
2. What subnet contains 192.168.10.77/26?
192.168.10.64/26.
3. Can :: appear twice in one IPv6 address?
No; only one zero run can be compressed that way.
4. What replaces ARP for IPv6 neighbor discovery?
ICMPv6 Neighbor Discovery.
5. Does an APIPA address prove the NIC is broken?
No. It often indicates failure to obtain DHCP configuration and needs layered diagnosis.