CS 453 Computer Networks Lecture 21 Layer 3 Network Layer Network Layer of the Internet.
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Transcript of CS 453 Computer Networks Lecture 21 Layer 3 Network Layer Network Layer of the Internet.
CS 453CS 453Computer NetworksComputer Networks
Lecture 21Lecture 21
Layer 3 Network LayerLayer 3 Network Layer
Network Layer of the InternetNetwork Layer of the Internet
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
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…
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
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
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
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
External IP/Port Internal IP
157.182.99.99/5050 10.0.0.15
157.182.99.99/5051 10.0.0.16
…. ….
IP Network Address Translation NAT
NAT
From: Tanenbaum, 2003, 446
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
connectionMust track state
Violates protocol layer conventionDips into IP header (port address)
Breaks on some protocols (payload imbedded addresses
Might not be able to scale to large subnet
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
ICMP
Message Meaning
Destination Unreachable Packet could not be delivered
Time Exceeded Timer/counter hit 0
Parameter Problem Invalid header info
Source quench Source swamping network
Redirect Suggests new routing
Echo Asks a destination to reply
Echo reply Reply from echo request
Timestamp request Asks for a reply with a timestamp
Timestamp reply Reply from a timestamp request
Finding Etherland
At Layer 3 we are dealing with, routing by, etc. IP and IP addressesBut within a LAN we are using the local Layer 2/1 technology…for example, ethernetWithin an Ethernet LAN, it sends, receives, routes ethernet frames, not IP packetsSo, when a packet containing an IP address hits an ethernet LAN,…How does it know where to go?
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---
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
RARP
Reverse Address Resolution Protocol What happens when you know the MAC
address but you need to know the IP address?