2.6.1. Address Structure

IPv4 Packet Header:

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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
/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)