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