- 32 bits = 4 octets. The mask says which leading bits are NETWORK and which trailing bits are HOST.
- Three notations for the same mask: dotted-decimal 255.255.255.0 = CIDR /24 = 24 ones then 8 zeros in binary.
- Classful baseline (still tested): A /8, B /16, C /24. CIDR/classless lets you use any prefix.
M-SUB 20% of exam domain 1.6–1.10 (within 1.0)
IP Addressing & Subnetting
The speed engine. Subnetting fast and correctly is the single make-or-break CCNA skill. Drill it until it is reflex.
- Block size = 256 − (the mask value in the "interesting" octet). E.g. /26 → mask .192 → block 64.
- Networks step by the block size: /26 → .0, .64, .128, .192. Your IP's network is the largest multiple of the block ≤ the octet.
- No scratch binary needed once you know block sizes - this is the exam speed engine.
- Network = first address of the block. Broadcast = last address (one below the next network).
- Usable hosts = 2^h − 2 (subtract network + broadcast), where h = 32 − prefix.
- First host = network + 1. Last host = broadcast − 1.
| Prefix | Mask | Block | Usable |
|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | .128 | 128 | 126 |
| /26 | .192 | 64 | 62 |
| /27 | .224 | 32 | 30 |
| /28 | .240 | 16 | 14 |
| /29 | .248 | 8 | 6 |
| /30 | .252 | 4 | 2 |
- Hosts → prefix: find smallest h with 2^h − 2 ≥ needed hosts, then prefix = 32 − h.
- Subnets → prefix: borrow ⌈log2(N)⌉ bits from the host portion.
- /30 (2 hosts) is the classic point-to-point link; /31 (RFC 3021) also works on p2p with no waste.
- List every subnet's host requirement (include /30 or /31 P2P links).
- Sort LARGEST first, allocate sequentially from the parent block, align each to its block boundary.
- Verify no overlap. This is the most-tested subnetting skill on the exam.
- RFC 1918 private: 10.0.0.0/8, 172.16.0.0/12 (172.16–172.31), 192.168.0.0/16.
- Special: APIPA 169.254.0.0/16 (= DHCP failure), loopback 127.0.0.0/8, CGNAT 100.64.0.0/10.
- Private addresses are translated to public by NAT to reach the Internet.
- 128-bit, written as 8 groups of 4 hex. Compression: drop leading zeros per group; replace ONE longest all-zero run with :: (only once).
- Standard prefix length is /64 for a subnet. Static config or SLAAC/EUI-64 for the interface ID.
- EUI-64: split the MAC, insert FFFE in the middle, flip the 7th bit (U/L) of the first byte.
- Global unicast 2000::/3 (Internet-routable). Unique local FC00::/7 (usually FD00::/8, private).
- Link-local FE80::/10 (every interface has one, not routed). Multicast FF00::/8. IPv6 has NO broadcast.
- Anycast = one address on multiple devices, nearest responds. Modified EUI-64 builds the host ID from the MAC.
09
1.10 Verify IP parameters on a client OS
- Windows: ipconfig /all (address, mask, gateway, DNS). macOS/Linux: ifconfig or ip addr.
- An address in 169.254.x.x means the client failed to get a DHCP lease (APIPA).