Page number	Title
1	Introduction
2	Connectionless service
3	Virtual packets
4	IP datagram format
5	Forwarding datagrams
6	Routing example
7	Routing table
8	Default routes
9	Routing tables and address masks
10	Address masks
11	Forwarding, destination address and next-hop
12	Best-effort delivery
13	IP datagram header format
14	IP datagram header fields
15	IP datagram options
16	Summary

• Formally, the unit of IP data delivery is called a datagram
• Includes header area and data area

• Datagrams can have different sizes
  • Header area usually fixed (20 octets) but can have options
  • Data area can contain between 1 octet and 65,535 octets (2¹⁶ - 1)
  • Usually, data area much larger than header

• For efficiency, information about forwarding is stored in a routing table
  • Initialized at system initialization
  • Must be updated as network topology changes
• Contains list of destination networks and next hop for each destination

• In practice, additional information is kept in routing table
• Destination stored as network address
• Next hop stored as IP address of router
• Address mask defines how many bits of address are in prefix
  • Prefix defines how much of address used to identify network
  • E.g., class A mask is 255.0.0.0
  • Used for subnetting

• To identify destination network, apply address mask to destination address and compare to network address in routing table
• Can use Boolean and

if ((Mask[i] & D) == Dest[i]) forward to NextHop[i]

• Consider 128.1.15.26: