4.3 The Internet Protocol (Part 1)

Internet's Network Layer Overview

  • The section is divided into two parts: IPv4 protocol and addressing, and NAT/IPV6.

Importance of Addressing

  • Addressing relates to relationships between ISPs, administrative boundaries, and technical operations.

IP Protocol Essentials

  • IP is concerning datagram formats, addressing, and packet handling conventions.

  • ICMP protocol is part of the network layer.

IP Datagram Format

  • Version Number: 4 bits specifying IP version.

  • Header Length: Indicates bytes in the header. Default is 20 bytes.

  • Total Length: Number of bytes in the datagram header + payload, max theoretical size is 64extKB64 ext{ KB}, but usually 1,500extbytes1,500 ext{ bytes}.

  • Type of Service Bits: Used for distinguishing datagram classes; most relevant for congestion notification.

  • TTL (Time to Live): Prevents infinite loops in routing, decremented at each router.

  • Upper Layer Protocol Field: Indicates protocol to receive payload (TCP, UDP).

  • Header Checksum: Recomputed at each router.

  • Source and Destination IP Addresses: 32 bits each.

IP Addressing Concepts

  • An IP address identifies an interface, not a host/router.

  • 32-bit IP addresses in dotted decimal notation (e.g. 223.1.1.1).

  • Subnet Definition: Devices in the same subnet can reach each other directly.

  • Subnet Components: Consists of subnet part (identical in interfaces) and host part (unique in interfaces).

  • Subnet Mask: Indicates the number of high-order bits in common.

  • CIDR Notation: Uses slash notation to specify address classes (e.g. 223.1.3/24).

IP Address Allocation

  • DHCP (Dynamic Host Configuration Protocol) automates IP address assignment.

  • Sequence of DHCP messages: Discover, Offer, Request, Acknowledgment.

  • Host requests and receives an IP from a DHCP server on joining a network.

Aggregation of Addresses

  • ISPs allocate address ranges from larger allocated blocks by ICANN, enabling address aggregation for efficient routing.

  • Longest Prefix Matching: Critical for routing decisions based on address prefixes.

ICANN and IPv4 Address Space

  • IPv4 address space is managed by ICANN and regional registries.

  • IPv4 space allocation is exhausted (as of 2011); shift to IPv6 anticipated due to address exhaustion issues.

Conclusion

  • Historical perspective on IPv4 addressing decisions by early internet architects highlights unexpected growth in internet connectedness.