Lecture 15

Internetworking:

Address Structure

Fragmentation and Reassembly

Protocol layers across an internetwork

IP-service model

• Datagram

• Best Effort

• Error reporting

IP-datagrams crossing a network with various MTU’s

Header Fields for IP over ethernet

IP-Address Classes

IP Addresses

Dotted Decimal Representation

• 01111111.00000101.11111111.00111111

• 255.255.255.255—broadcast address

• 0.0.0.0-forbidden

Hierarchy and Network Classes

Datagram Forwarding

• Host and routers with same network address can communicate directly

• Routers exchange packets between networks

if(NetworkNum of dest.=NetworkNum of one of my interfaces) then deliver packet to destination over that interface

else

if (NetworkNium of dest. is in my forwarding table) then deliver packet to NextHop router

else

deliver packet to default router

Forwarding table for R2

NetworkNum NextHop

1 R3

2 R1

3 Interface 1

4 Interface 0

Local Delivery Address Translation

• Address Resolution Protocol (ARP)• ARP request induces the target to broadcast

its IP and Local address across the LAN. Requesting router then knows how to deliver the message

• Target keeps the senders local address and IP address in its own ARP table—in case it needs to ACK

Host Configuration

• Dynamic Host Configuration Protocol (DHCP)

• DHCP Server-assigns dynamic IP addresses• New hosts broadcast DHCPDISCOVER

message to 255.255.255.255-this message does not go to other networks

• Request may use a DHCP Relay Agent to reach a server on another network

Error Reporting

• Internet Control Message Protocol (ICMP)– Host unreachable– Reassembly failed– TTL=0– IP header checksum failed– ICMP-Redirect

Virtual Private Network

Tunnel Concept

Forwarding table for router R1

NetworkNum NextHop

1 Interface 0

2 Virtual interface 0

Default Interface 1