Network Layer

Peter Wood

The Internet Concept (1)

The internet concept

The Internet Concept (2)

The internet concept

Routers (1)

Networks connected by a router

Routers (2)

An internet

Hosts and Routers

The network layer

Forwarding and Routing

IP Addresses and Dotted-Decimal Notation

Example addresses

IP Addressing

Interface addresses



Subnet Addressing

Classful Addressing

Obtaining a Block of Addresses

Special IP Addresses

Special IP addresses

Obtaining a Host Address

Dynamic Host Configuration Protocol

DHCP client-server scenario

DHCP Client-Server Interaction

For a newly arriving host, DHCP is a four-step process:

DHCP client-server interaction

Network Address Translation

NAT Example

Network address translation

Criticisms of NAT

IP Datagrams

Forwarding IP Datagrams

Extracting Network Prefixes

Best-Effort Delivery

IP Datagram Format (1)

IP datagram header

IP Datagram Format (2)

IP datagram header


Transmission across an Internet

Traversing different networks

Example of IP Header

Packet capture showing IP header

Maximum Transmission Unit (MTU)

Datagram Fragmentation (1)


Datagram Fragmentation (2)

Datagram Reassembly

Fragment Loss

Internet Control Message Protocol (ICMP)

ICMP Message Types

ICMP messages

Use of ICMP Messages

Routing Algorithms

Internet Routing Protocols

Abstract Graph Model

Open Shortest Path First

OSPF Algorithm

 1.  N' = {u}
 2.  for all nodes v
 3.    if v is a neighbour of u
 4.       then D(v) = c(u,v)
 5.       else D(v) = infinity

 6.  Loop
 7.    find w not in N' such that D(w) is a minimum
 8.    add w to N'
 9.    update D(v) for each neighbour v of w not in N':
10.      D(v) = min( D(v), D(w) + c(w,v) )
11.  until N' = N

Example of OSPF Algorithm

Links to more information

See Chapter 4 of [Kurose and Ross], Chapters 20, 21, 22 and parts of 23 of [Comer] and parts of Chapter 5 of [Tanenbaum].