6.2IT POSSIBLE TO BOOT A MACHINE AND ACCESS THE NETWORK WITHOUT CONF...
2.6.2it possible to boot a machine and access the network without configuring any protocolinformation.This eliminates many of the manual configuration issues, such as using thewrong subnet mask, duplicate IP addresses, and limited or no host name resolution. Someof the items to consider when you implement and use DHCP are lease time, number ofhosts in a scope, network traffic, scope options, and topology.When a machine acquires an IP address from a DHCP server, it acquires a lease.Therequest for the lease is a message called a DHCPREQUEST, which is broadcast by theDHCP client looking for DHCPOFFERs of a lease from a DHCP server.The lease durationfor a DCHP address is specified in the scope set on the server and defaults to eight days. At50 percent of the lease duration, the DCHP client sends a directed request to the DHCPserver that issued the lease and requests a renewal of the lease. If no DHCPACK (acknowl-edgment) is received from the server, the DHCP client waits until 87.5 percent of the leasetime, and then makes a final request to renew the IP address. If no DHCPACK is receivedat this point, the client waits until the lease is expired and starts the process over. If aDHCP client is unable to receive an IP address lease, it will use an alternate configuration,if one is specified. If there is no alternate configuration, the client will use APIPA to startthe TCP/IP services and assign itself an address from the APIPA pool (169.254.0.0/16).To determine the appropriate lease time for your network, you should consider the fol-lowing:
■
Number of hosts If the number of hosts is close to the number of total IPaddresses in your DHCP server’s scope, the lease should be shorter—about threedays. If there are a great deal more IP addresses than hosts, a longer lease can beassigned.■
Mobile users If you have a small number of mobile users and the clientmachines do not frequently move from one network to the other, a longer leaseduration is recommended. Conversely, if you have more mobile users, a shorterlease will be preferred, so that the IP addresses will be released sooner andreturned to the available pool of addresses.■
Unlimited It is possible to set the lease duration to unlimited, but it presents achallenge if you wish to change the DHCP settings, since this setting requires theclient to initiate the DHCPREQUEST.Because they are broadcast, the DHCPREQUEST messages do not cross routerboundaries, unless the router is capable of forwarding DHCP broadcast messages, in com-pliance with RFC 2131.You can also configure a DHCP relay to forward the requests to aDHCP server.Using DHCP can reduce IP address conflicts by preventing the need for static IPaddress. It also can eliminate invalid subnet masks, since they are also assigned by theDHCP server. Another advantage is the use of scope properties. By assigning scope proper-ties, you can define default gateways, DNS servers,WINS servers, and the type of name res-olution that is preferred. By managing name resolution settings, you can help eliminatebroadcast traffic.Transitioning to IPv6
IPv6, defined in RFC 2460, is now production-ready to use on most operating system plat-forms. At this point, it is still early in the transition from IPv4.The change to IPv6 will takesome time, but with each day, it becomes more necessary due to the growing shortage ofIPv4 addresses. Although the larger address space is the most immediate need, IPv6 offersother advantages over IPv4, including the following:■
Better security (built in support for IPSec)■
Support for both stateful and stateless address configuration■
An efficient hierarchical routing infrastructure■
A new header format that provides lower overhead■
Neighbor Discovery (ND) for managing nodes on the same link, replacing ARP,ICMPv4 router discovery, and ICMPv4 redirect messages■
Virtually unlimited extension headers (in comparison to IPv4’s limit of 40 bytes)■
Quality of service (QoS) related header fieldsThe utilities and concepts associated with IPv6 are similar to IPv4, but not identical. Inthe following sections, we’ll take a look at how to install IPv6 and start to familiarize our-selves with the new utilities used to manage it.IPv6 on Windows Server 2003 provides a new header format that is streamlined tominimize overhead and provide more efficient processing while crossing intermediaterouters. All the option fields and any other fields in the header that are not required forrouting are placed after the IPv6 header.The IPv6 header also added more QoS support byadding Flow Label fields that provide special handling for a series of packets that travelbetween a source and destination.ND is a set of process and messages that are used in an IPv6 environment to identifyrelationships between neighboring nodes.This allows hosts to discover routers on the samesegment, addresses, and address prefixes.With ND, hosts can also resolve neighboring nodesand determine when the MAC address of a neighbor changes (similar to ARP in IPv4).ND also provides the process for address autoconfiguration, also referred to as statelessDHCP version 6 (DHCPv6) protocol server, ND provides a complex process that allowseach interface to use router advertisement messages to define an IPv6 address, and thensubsequently ensure the uniqueness of the selected address. Currently, the standards forDHCPv6 and IPv6 stateful addressing are still under development, so neither feature is sup-ported on Windows XP/Server 2003 products at this time.The new routing structure provides a hierarchical addressing and routing structure thatincludes a global addressing scheme. Global addresses are the equivalent of public IPv4addresses and are accessible over the Internet.The global addressing scheme defines newways to summarize global addresses to facilitate smaller routing tables on the Internet back-bone, thus improving the efficiency and performance on the Internet.