CS 453 Computer Networks

1. CS 453Computer Networks Lecture 21 Layer 3 Network Layer Network Layer of the Internet

2. IP Network Address Translation NAT NAT Remember that IP addresses should be unique – Globally, Universally There are not really enough IP addresses to go around Many situations where you have a single “legitimate” IP address So, how do you expand your local area network with that single address Network Address Translation - NAT

3. IP Network Address Translation NAT NAT Common scenario— You buy a cheap ethernet switch/router from Nerd-R-Us to install at home Your connect it to your Cable modem Your router acts like a single DHCP client – gets a single IP address from the ISP So, you hook up your mother-in-law’s computer, Uncle Fred’s computer, and you throw a Cat5 cable out the window to your neighbor and connect his computer to your router Ok, but…

4. IP Network Address Translation NAT NAT Common scenario— … you can’t all use the same ISP assigned IP address… How would packets know which host they are going to, coming from Network Address Translation

5. IP Network Address Translation NAT NAT There are three special IP address ranges Non-routable Non-unique These are 10.x.x.x 192.168.x.x ??? So the “backside” of the router issues DHCP addresses too,… …but these addresses are from the non-routable families

6. IP Network Address Translation NAT NAT Recall that the IP packet header contains the IP address of the host, . … but also the socket that the transport layer is using From the outside all hosts in our little LAN look like they have the same IP address

7. IP Network Address Translation NAT NAT The router creates a table… The router assigns each internal host a port number, along with the The table maps the external IP+port to the internal IP in the local LAN

8. IP Network Address Translation NAT NAT From: Tanenbaum, 2003, 446

9. IP Network Address Translation NAT NAT NAT really irks a lot in the IP community Violates the IP unique address rule IP is connectionless, NAT creates a connection Must track state Violates protocol layer convention Dips into IP header (port address) Breaks on some protocols (payload imbedded addresses Might not be able to scale to large subnet

10. ICMP Internet Control Message Protocol In addition to all the user, host to host message traffic using IP protocol in the network… Routers, switches and other devices need to talk to each other …to keep the network running They have their own protocol for doing this ICMP

11. ICMP

12. Finding Etherland At Layer 3 we are dealing with, routing by, etc. IP and IP addresses But within a LAN we are using the local Layer 2/1 technology …for example, ethernet Within an Ethernet LAN, it sends, receives, routes ethernet frames, not IP packets So, when a packet containing an IP address hits an ethernet LAN,… How does it know where to go?

13. ARP-Address Resolution Protocol Once on the LAN the frames address destination address must be set to the MAC address of the destination host interface To do this---

14. ARP-Address Resolution Protocol The sending machine Reads the IP address from the message Transmits an ARP request – “Who does this belong to” The IP address owner replies “Me” by returning its MAC address The sender assigns the received MAC to the destination field in the frame header… …and send the frame on its way

15. RARP Reverse Address Resolution Protocol What happens when you know the MAC address but you need to know the IP address?