Small Office Service

1
Small Office Service

• Serial Router Connects Internal Stations to
  Shared Broadband Access Service

Serial Router
ISP
Shared Broadband Line
Small Office
2
Small Office Service

• Serial Routers are Very Simple
• One or more ports to connect the serial port to
  the LAN
• One port to connect to the high-speed access
  line, which is serial
• Much less expensive than general-purpose routers
  because no complex router forwarding decision
  caused by multiple possible output ports

Serial Router
3
Small Office Service

• Serial Router Connects Internal Stations to
  Shared Broadband Access Service
• Companies with several stations pay much more for
  line than when connecting a single PC
• Each machine gets its own IP address via DHCP
• Serial router may be a user PC with added
  software
• Often, router is called a gateway (the old name
  for router)

4
Connecting Two PCs at Home

• Physically, One PC (the Gateway PC) Connects to
  the Hub or Switch and to the Broadband Line
• Gateway (router) PC needs two Network Interface
  Cards, one for hub, one for broadband modem

Hub or Switch
Broadband Line to ISP
NIC
ADSL/Cable Modem
Gateway PC
5
Connecting Two PCs at Home

• Gateway PC Needs Gateway Software
• On NIC to broadband line, acts like a single
  station
• Gets temporary IP address from ISP DHCP
• Sends and receives to ISP using this IP address

Send, Receive with Temporary IP Address
Gateway PC
6
Connecting Two PCs at Home

• Gateway PC Needs Gateway Software
• Each PC is set up with a permanent IP address
• So gateway PC has two addresses, one bound to
  each NIC

Gateway Program
Temporary IP Address
Permanent IP Address
Permanent IP Address
7
Connecting Two PCs at Home

• Gateway Software Does Network Address Translation
  (NAT)
• When a PC sends using permanent IP address
• Gateway software replaces source IP permanent
  address with temporary IP address ISP expects

Gateway PC
Temporary IP Address
Permanent IP Address
Permanent IP Address
8
Connecting Two PCs at Home

• Gateway Software Does Network Address Translation
  (NAT)
• When a PC receives
• Gateway software replaces ISP temporary IP
  destination address with receivers permanent IP
  address, passes on to PC

Gateway PC
Temporary IP Address
Permanent IP Address
Permanent IP Address