IPv4 & IPv6 Addressing

Layer 3 defines logical addressing and routing. Understanding IP headers, subnetting, and MTU is critical for network engineering.

1. The IPv4 Header

The standard IPv4 header is 20 bytes (without options). Routers process this header to make forwarding decisions based on the Destination IP.

Ver/IHL 0x45 1B

TOS DSCP 1B

Total Length len(pkt) 2B

ID/Flags/Frag Fragment Info 4B

TTL 64 1B

Proto TCP=6 1B

Source IP 192.168.1.10 4B

Dest IP 8.8.8.8 4B

TTL (Time To Live): Decremented by 1 at every router hop. Prevents infinite loops. When it hits 0, the router sends an ICMP "Time Exceeded" message.
Protocol Field: Tells IP what payload is inside.
- 1: ICMP
- 6: TCP
- 17: UDP
- 89: OSPF

2. Historical Context: Classful Addressing

Before CIDR (1993), IP addresses were divided into strict "Classes" based on the first octet. This wasted millions of addresses.

Class A (0-127): /8 (16M hosts). Only 126 networks existed. (e.g., Apple, IBM, MIT).
Class B (128-191): /16 (65k hosts). Too big for most companies.
Class C (192-223): /24 (254 hosts). Too small.

Subnetting & CIDR Cheat Sheet

CIDR (Classless Inter-Domain Routing) allows variable-length subnet masks (VLSM), breaking the rigid class structure.

RFC 1918 Private Address Space

These ranges are not routable on the public Internet. NAT (Network Address Translation) is required.

Class A: 10.0.0.0/8 (10.0.0.0 - 10.255.255.255)
Class B: 172.16.0.0/12 (172.16.0.0 - 172.31.255.255)
Class C: 192.168.0.0/16 (192.168.0.0 - 192.168.255.255)

IPv6 Address Types

Global Unicast (2000::/3): Publicly routable (like IPv4 Public IP).
Link-Local (FE80::/10): Only valid on the local segment. Used for neighbor discovery and routing protocol hellos. (Always exists!).
Unique Local (FC00::/7): Private use (like IPv4 RFC 1918).
Multicast (FF00::/8): One-to-Many.

CIDR	Subnet Mask	Hosts	Magic Number (Block Size)
/24	255.255.255.0	254	1 (in 3rd octet)
/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 (Common for P2P links)
/32	255.255.255.255	1	1 (Host route)

Engineer's Notebook: The /31 Subnet

Historically, /30 was the standard for point-to-point links (4 IPs, 2 usable). Modern routers support /31 (2 IPs total, both usable as host addresses) to conserve address space on P2P links. RFC 3021.

3. MTU & Fragmentation

MTU (Maximum Transmission Unit) is typically 1500 bytes for Ethernet. If a packet is larger than the MTU:

If DF (Don't Fragment) bit is Set: The router drops the packet and sends ICMP Type 3 Code 4 ("Fragmentation Needed").
If DF is Clear: The router fragments the packet, increasing CPU load and overhead.

Path MTU Discovery (PMTUD) relies on ICMP. If you block all ICMP at your firewall, PMTUD breaks, causing "black hole" connections where small packets pass but large ones (like HTTP responses) fail.