The Internet Protocol (IP) is the network-layer protocol that provides logical addressing and routing across interconnected networks. While Ethernet handles delivery within a single LAN segment, IP enables end-to-end delivery across the entire internet by assigning each host a globally meaningful address and routing packets hop-by-hop through routers.

IPv4 Header

The IPv4 header is 20 bytes minimum (without options) and contains the following critical fields:

Field	Size	Purpose
Version	4 bits	Always 4 for IPv4
IHL (Internet Header Length)	4 bits	Header length in 32-bit words (min 5 = 20 bytes)
Total Length	16 bits	Entire packet size (header + payload), max 65,535 bytes
TTL (Time to Live)	8 bits	Decremented by 1 at each router; packet dropped when TTL reaches 0 — prevents infinite routing loops
Protocol	8 bits	Identifies the transport layer (6 = TCP, 17 = UDP, 1 = ICMP)
Header Checksum	16 bits	Error detection for the header only (not the payload)
Source IP	32 bits	Sender's IP address
Destination IP	32 bits	Receiver's IP address

Additional fields include Identification, Flags (including the DF — Don't Fragment flag), and Fragment Offset, which are used for fragmentation.

Subnetting and CIDR

IP addresses are divided into a network prefix and a host identifier. CIDR (Classless Inter-Domain Routing) notation expresses this split: 192.168.1.0/24 means the first 24 bits are the network prefix, leaving 8 bits for hosts — that is 2^8 = 256 addresses (254 usable, since the first is the network address and the last is broadcast).

Common subnet sizes:

• /24 — 256 addresses (typical small LAN)
• /16 — 65,536 addresses (large campus)
• /8 — 16,777,216 addresses (very large organization)

Private Address Ranges

Three ranges are reserved for private networks (not routable on the public internet):

• 10.0.0.0/8 — 16.7 million addresses
• 172.16.0.0/12 — 1 million addresses (172.16.0.0 to 172.31.255.255)
• 192.168.0.0/16 — 65,536 addresses

These are used behind NAT (Network Address Translation), where a router maps many private addresses to one or a few public addresses. NAT conserves the limited IPv4 address space and provides a layer of security by hiding internal addresses.

IPv6

IPv6 was designed to solve IPv4 address exhaustion. Key differences:

• 128-bit addresses — 3.4 * 10^38 possible addresses (effectively unlimited)
• Simplified header — fixed 40 bytes, no header checksum (upper layers handle integrity), no options field (replaced by extension headers)
• No router fragmentation — only the source host may fragment; routers send "Packet Too Big" ICMPv6 messages instead (Path MTU Discovery)
• Mandatory features — IPsec support, multicast, neighbor discovery (replaces ARP)

An IPv6 address looks like 2001:0db8:85a3:0000:0000:8a2e:0370:7334, with consecutive zero groups abbreviated using ::.

Fragmentation

When an IPv4 packet is larger than the MTU of the next link, the router fragments it into smaller packets, each with its own IPv4 header. The Identification field ties fragments together; the Fragment Offset field positions each piece; the More Fragments (MF) flag indicates whether more pieces follow. Fragmentation is costly — it increases overhead, wastes bandwidth, and can cause retransmission of the entire original packet if any fragment is lost. Setting the DF (Don't Fragment) flag forces routers to drop oversized packets and send an ICMP "Fragmentation Needed" message, enabling Path MTU Discovery.

Routing: Longest Prefix Match

Routers maintain a routing table mapping destination prefixes to next-hop addresses. When a packet arrives, the router finds the entry whose prefix matches the most bits of the destination IP — this is the longest prefix match. For example, if the table contains both 10.0.0.0/8 and 10.1.0.0/16, a packet destined for 10.1.2.3 matches the /16 route because it is more specific.