计算机网络PPT电子教案-第6章 网络层.ppt

* * * * * * * * * * * 6.9.3 ICMP差错控制 ICMP差错报告报文： 目的站不可达 源站抑制、 超时 参数问题 改变路由 * 目的站不可到达 网络不可到达（net unreachable） 主机不可到达（host unreachable） 协议不可到达（protocol unreachable） 端口不可到达（port unreachable） 源路由选择不能完成（source route failed） 目的网络不可知（unknown destination network） 目的主机不可知（unknown destination host） ? 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. * 23.11 Dynamic Host Configuration Protocol (DHCP) When a computer boots the client computer broadcasts a DHCP Request the server sends a DHCP Reply DHCP uses the term offer to denote the message a server sends and we say that the server is offering an address to the client We can configure a DHCP server to supply two types of addresses: permanently assigned addresses as provided by BOOTP or a pool of dynamic addresses to be allocated on demand Typically, a permanent address is assigned to a server, and a dynamic address is assigned to an arbitrary host In fact, addresses assigned on demand are not given out for an arbitrary length of time ? 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. * 23.11 Dynamic Host Configuration Protocol (DHCP) DHCP issues a lease on the address for a finite period The use of leases allows a DHCP server to reclaim addresses When the lease expires the server places the address to the pool of available addresses this allows the address to be assigned to another computer When a lease expires, a host can choose to relinquish the address or renegotiate with DHCP to extend the lease Negotiation occurs concurrent with other activity Normally, DHCP approves each lease extension A computer continues to operate without any interruption However, a server may be configured to deny lease extension for administrative or technical reasons DHCP grants absolute control of leasing to a server If a server denies an extension request the host must stop using the address ? 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. * 23.12 DH