DHCP, NAT

DHCP

• Dynamic Host Configuration Protocol(动态主机配置协议) (RFC 2131)
• Bootstrap Protocol BOOTP(引导程序协议) allows a diskless(无盘) workstation to be remotely booted up in a network
  • UDP port 67 (server) & port 68 (client)
• DHCP builds on BOOTP to allow servers to deliver configuration information to a host
  • Used extensively to assign temporary IP addresses to hosts
  • Allows ISP(Internet Service Provider)(互联网服务提供商) to maximize usage of their limited IP addresses
  • Time thresholds(阈值) to enforce lease time(增加租赁时间)

Network Address Translation (NAT)

• Class A, B, and C addresses have been set aside for use within private Internets
  • Private IP addresses are sufficient for use inside of private networks
  • But packets with private (“unregistered”) addresses are discarded by routers in the global Internet
• NAT (RFC 1631): method for mapping packets from hosts in private internets into packets that can traverse(穿过) the Internet
  • A device (computer, router, firewall) acts as an agent between a private network and a public network
  • A number of hosts can share(共享) a limited number of registered IP addresses

Placement of Operation of a NAT Box

• NAT: provides mapping between public IP address and private IP addresses
  

NAT Operations

• Hosts inside private networks generate packets with private IP address & TCP/UDP port #s
• NAT maps each private IP address & port # into shared global IP address & available port #
• Translation table allows packets to be routed unambiguously(明确地)

NAT Discussions

• In theory, up to 2^16 private IP addresses supported by a single public IP address in NAT box
• Overhead in NAT operation
• TCP/UDP Port number used for NAT mapping at IP layer, violating OSI layer architecture principle
• The principle is that a higher layer utilize a service provided by the lower lever but not vice versa(反).