Work of DHCP Dynamic Host Configuration Protocol and PAT Port Address Translation

DHCP – Dynamic Host Configuration Protocol

  • DHCP server is configured with a range of addresses
  • Host 3 boots up and enters Initialize state
  • It broadcasts a DHCP Discover message
  • Then enters Select state
  • DHCP servers send DHCP offer message
  • Host 3 sends a DHCP request to one of the servers
  • Host 3 enters the Request state
  • DHCP server responds with a DHCP ack (with address)
  • Host 3 enters the Bound state, accepting the lease

Network Address Translation – NAT

Operates at the edge of a company network, Converts the private IP addresses into one or more public addresses

Several flavours:

  • Fixed private to public relationship – Static NAT
  • Demand-based – when a private address needs external access, an address is taken from a pool of public ones and the conversion done at the edge router – Dynamic NAT
  • Overload NAT – also called PAT – Port Address Translation

dhcp in ip address

PAT – Port Address Translation

  • User on IP Address ‘p’ and port ‘r’ sends packet for the outside
  • Router converts this to IP address ‘w’ and port 36578
  • Reply comes back for ‘w’ and 36578
  • Router converts this to address ‘p’ and port ‘r’
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