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2.6.1. Address Structure
IPv4 Packet Header:
The TTL field decrements at each hop (router)—when it reaches 0, the packet is dropped and an ICMP "Time Exceeded" is sent back. This is how traceroute works.
IPv4 Address Structure
IP Address: 192.168.1.100
Binary: 11000000.10101000.00000001.01100100
Subnet Mask: 255.255.255.0 (or /24)
Binary: 11111111.11111111.11111111.00000000
|------Network------|--Host--|
Subnetting Quick Reference
CIDR to Subnet Mask:
| CIDR | Subnet Mask | Hosts per Subnet | Block Size |
|---|---|---|---|
| /22 | 255.255.252.0 | 1,022 | 4 (third octet) |
| /23 | 255.255.254.0 | 510 | 2 (third octet) |
| /24 | 255.255.255.0 | 254 | 256 |
| /25 | 255.255.255.128 | 126 | 128 |
| /26 | 255.255.255.192 | 62 | 64 |
| /27 | 255.255.255.224 | 30 | 32 |
| /28 | 255.255.255.240 | 14 | 16 |
| /29 | 255.255.255.248 | 6 | 8 |
| /30 | 255.255.255.252 | 2 | 4 |
Formula: Hosts per subnet = 2^(host bits) - 2 (subtract network and broadcast)
Classful ranges (still tested, though routing is classless today) are set by the first octet: Class A 1–126 (default /8), B 128–191 (/16), C 192–223 (/24), D 224–239 (multicast), E 240–255 (experimental); 127 is reserved for loopback.
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Written byAlvin Varughese
Founder•20 professional certifications