NAT

1
NAT

• Network Address Translation.
• Converts private LAN IP address to public WAN IP
  address.
• Requires a dynamic table of source port numbers
  to direct incoming IP traffic.

Source - 192.168.0.4 2345 Destination -
223.24.123.24 80 Gateway - 192.168.0.1
Source - 243.68.73.1 2345 Destination -
223.24.123.24 80
LAN IP 192.168.0.1
WAN IP 243.68.73.1
192.168.0.4
223.24.123.24
NAT
2
Router

• Determines the next network to which an IP packet
  should be forwarded toward its destination
• The router is connected to at least two networks
  and decides which way to send each information
  packet.

Router
Ethernet (LAN)
Ethernet (WAN)
Ethernet (LAN)
3
Residential Gateway

• Usually a router, DHCP server, and NAT
• Some Residential Gateways are wireless and
  therefore also a bridge
• Shares Broadband or Dial-up connection
• No need for multiple IP addresses
• All PCs can be on-line at same time

Ethernet (LAN)
Ethernet (WAN)
Ethernet (LAN)
Residential Gateway
4
Firewall
5
Network Layers
Application
Web browsing (HTTP)Email (SMTP)
Application Client/Server
Presentation
Session
Transport
TCP / UDP
Reliable flow
Network
IP
Packet routing
Data-link
Ethernet MAC address
Device Drivers Network cards Hubs
Physical
6
Protocol Stack

• Total 54 byte overhead
• Ethernet (MAC)
• IP (addresses)
• TCP (seq ack)