ptg NetBIOS Name Resolution 209 As shown in Figure 11.11, the WINS tab lets you manually add the addresses of WINS servers. You can also enable LMHosts lookup and import an existing LMHosts file. Note that, by default, the system receives a NetBIOS setting from the DHCP server, but you can also elect to override the DHCP setting by enabling or disabling NetBIOS over TCP/IP. When a WINS client computer boots after being configured to use WINS, the follow- ing process occurs: 1. Service startup—As the computer boots, various services are started, some of which need to be made known to other computers. 2. Registration request—To be known to other computers on the network, the service must register. A WINS client computer packages the NetBIOS name and the computer’s IP address inside a name registration request, and the reg- istration request is sent to the WINS server. Upon receiving the registration request, WINS checks its database to see whether the name is already registered. If the name does not exist, WINS adds the NetBIOS name and IP address pair to its database and sends a name registration response indicating the name was successfully registered. If the requested NetBIOS name already exists in FIGURE 11.11 Configuring WINS in Windows Vista. From the Library of Athicom Parinayakosol ptg 210 HOUR 11: Name Resolution the WINS database, WINS challenges the computer currently registered by sending a message to the registered IP address. If the currently registered com- puter responds, a negative acknowledgment is sent to the computer attempt- ing to register the name. If the computer being challenged doesn’t respond, WINS allows the registration to occur and overwrites the previous registration. 3. Lease—Assuming the computer is successful in registering its NetBIOS names and services with WINS, these names are considered leased. In essence, the computer is allowed to use the NetBIOS name for a specified period of time— for instance six days—but the client can renew the lease before it expires. The client typically renews the lease at 50% of the total lease time or in this case every three days. Earlier I noted that the 16th character of a NetBIOS name is not configurable by the user. During the WINS registration process, the 16th character is appended to the name by the WINS server based on what type of service the computer is trying to register before it is placed in the database. Between computer names, workgroup names, and a number of services, it is not unusual for a single computer to have 5 to 10 registration entries in the WINS database. As another example of the WINS name resolution process, suppose a user on a com- puter uses a utility such as Network Neighborhood to connect to another computer on the network. A name query request, which includes the desired NetBIOS name, is constructed by the application and sent to the WINS server. When WINS receives the request, it queries its database for a matching registration. If the requested name is found, WINS returns the corresponding IP address in the response packet. After the client computer has the IP address for the requested computer, the client can then communicate directly. Testing NetBIOS Name Resolution You can test NetBIOS name resolution using NetBIOS-based utilities. One typical test of name resolution is using the net view command, which enables you to view the share point names on a server. (Remember that a share point is a directory where client computers can connect with another computer to view or exchange files.) To perform this test, choose a computer that has one or more share points. At a com- mand prompt, type net view \\computername where computername is the name of the computer you selected. If net view is capa- ble of resolving the computer name to an IP address, you should see the names of share points listed in the first command and response. From the Library of Athicom Parinayakosol ptg Q&A 211 You can also use the ubiquitous Ping utility to test NetBIOS name resolution. On most Windows systems, if NetBIOS name resolution is working properly, you should be able to ping a computer by its NetBIOS computer name. For instance, if a com- puter has the computer name Shirley, you should be able to type ping Shirley and receive a response. Summary Name resolution enables the use of meaningful, easy-to-remember names for com- puters instead of the IP address assigned to a computer. This hour described name resolution by hostname and also through DNS. You also learned about the NetBIOS name resolution system used on Microsoft networks. Q&A Q. What is a domain name? A. A domain name is a name used to identify a network. The domain name is administered by a central authority to ensure the name’s uniqueness. Q. What is a hostname? A. A hostname is a single name that is assigned to a particular host and mapped to an IP address. Q. What is an FQDN? A. A combination of a hostname concatenated to a domain name by the addi- tion of a dot character. For example, a hostname bigserver and a domain name mycompany.com when combined become the FQDN bigserver.mycompany.com. Q. What are DNS resource records? A. Resource records are the entries contained in a DNS zone file. Different resource records are used to identify different types of computers or services. Q. What type of resource record is used for an alias? A. CNAME; it is used to map an alias to the name specified in an A record. From the Library of Athicom Parinayakosol ptg 212 HOUR 11: Name Resolution Q. How do you centrally administer entries in an LMHosts file? A. You can implement centralized administration by adding an include state- ment to the LMHosts file. A line that starts with #INCLUDE and provides the location of an LMHosts file located on a server provides a link to the central file. Q. How can you create static NetBIOS entries in the NetBIOS name cache? A. By using the keyword #PRE on the line of the desired entry in an LMHosts file. Workshop . At the command line of your computer, enter the command ping localhost and write down the IP address that you see. . At the command line of your computer, enter the command hostname and write down the hostname that is returned. . Enter a ping command followed by the hostname for your computer. . If your computer has a domain name, ping your FQDN. . Determine whether IP is configured to use a DNS server. If so, try the following pings: ping www.internic.net ping www.whitehouse.gov . Use NSLookup to connect to one of your ISP’s DNS servers. Key Terms Review the following list of key terms: . DNS (domain name system)—A system for naming resources on TCP/IP net- works. . Domain name—A name assigned to a hierarchical partition of the DNS namespace. . FQDN (fully qualified domain names)—The name generated by concatenat- ing a hostname with a domain name. From the Library of Athicom Parinayakosol ptg Key Terms 213 . Hostname—A single name used to identify a computer (host). . LMHosts—A file that associates IP addresses to NetBIOS names. . Resource record—An entry added to zone files. There are a number of resource record types, and each type has a specific purpose. . WINS (Windows Internet Naming Service)—A WINS server is a Microsoft implementation of a NetBIOS name server. . Zone file—The configuration files used by DNS servers. These text files are used to configure DNS servers. From the Library of Athicom Parinayakosol ptg This page intentionally left blank From the Library of Athicom Parinayakosol ptg HOUR 12 Automatic Configuration What You’ll Learn in This Hour: . Dynamic address assignment . DHCP . Network Address Translation . Zeroconf In the old days, every client computer held a static IP address defined somewhere within a configuration file, and to change the configuration, the system administrator had to go change the file. Networks today, however, require a more versatile and convenient approach. Why not manage the TCP/IP configuration through automated services run- ning on the network? This hour looks at some common techniques for automating TCP/IP address assignment. At the completion of this hour, you will be able to . Describe DHCP and the benefits it provides . Describe the process of leasing an IP address through DHCP . Describe the purpose of Network Address Translation . Show how computers use the zero configuration protocols The Case for Server-Supplied IP Addresses Every computer, as you learned in a previous hour, must have an IP address to operate on a TCP/IP network. The IP addressing system was originally designed for the logical condi- tion in which each computer is preconfigured with an IP address. This condition is known From the Library of Athicom Parinayakosol ptg 216 HOUR 12: Automatic Configuration as static IP addressing. Each computer knows its IP address from the moment it boots and is able to use the network immediately. Static IP addressing works well for small, permanent networks, but on larger networks that are subject to reconfigura- tion and change (such as new computers coming and going from the network), static IP addressing has some limitations. The principal shortcomings of static IP addressing are . More configuration—Each client must be configured individually. A change to the IP address space or to some other parameter (such as the DNS server address) means that each client must be reconfigured separately. . More addresses—Each computer uses an IP address whether it is currently on the network or not. . Reduced flexibility—A computer must be manually reconfigured if it is assigned to a different subnetwork. As an answer to these limitations, an alternative IP addressing system has evolved in which IP addresses are assigned upon request using the DHCP protocol. DHCP was developed from an earlier protocol called BOOTP, which was used primarily to boot diskless computers. (A diskless computer receives a complete operating system over the network as it boots.) DHCP has become increasingly popular in recent years because of the dwindling supply of IP addresses and the growth of large, dynamic networks. It is quite likely that the majority of all computers with Internet access receive their configurations through DHCP. The small router/firewall device that brings the Internet to your home network is probably also acting as a DHCP server. What Is DHCP? DHCP is a protocol used to automatically assign TCP/IP configuration parameters to computers. DHCP is a standard described in RFC 1531. Other RFCs—1534, 1541, 2131, and 2132—address enhancements and specific vendor implementations of DHCP. A DHCP server can supply a DHCP client with a number of TCP/IP settings, such as an IP address, a subnet mask, and the address of a DNS server. Because the DHCP server is assigning the IP addresses, only the DHCP server must be configured with static IP address information. The only networking parameter you need to configure on the client end is an option for the client to receive IP address information from a DHCP server. The rest of the TCP/IP configuration is From the Library of Athicom Parinayakosol ptg How DHCP Works 217 transmitted from the server. If some aspect of the TCP/IP configuration changes on the network, the network administrator needs only to update the DHCP server, rather than updating each client manually. Furthermore, each client receives a lease of finite duration for the address. If the client is no longer using the address when the lease expires, the address can be assigned to another client. The effect of DHCP’s leasing feature is that, typically, a network will not need as many IP addresses as it has clients. DHCP is especially important in today’s environment, in which many employees carry notebook computers between offices of a large corporation. If a laptop com- puter is configured with a static IP address, it must be reconfigured each time the traveling employee plugs into a different network. If the computer is configured to receive an IP address through DHCP, the laptop automatically receives a complete TCP/IP configuration each time the user attaches to a network with a DHCP server. How DHCP Works When a DHCP client computer is started, the TCP/IP software is loaded into memory and starts to operate. However, because the TCP/IP stack has not been given an IP address yet, it is incapable of sending or receiving directed datagrams. The computer can, however, transmit and listen for broadcasts. This capability to communicate via broadcasts is the basis for how DHCP works. The process of leasing an IP address from the DHCP server involves four steps (see Figure 12.1): 1. DHCPDISCOVER—The DHCP client initiates the process by broadcasting a datagram destined for UDP port 68 (used by BOOTP and DHCP servers). This first datagram is known as a DHCP Discover message, which is a request to any DHCP server that receives the datagram for configuration information. The DHCP discover datagram contains many fields, but the one that is most important contains the physical address of the DHCP client. 2. DHCPOFFER— A DHCP server configured to lease addresses for the network on which the client computer resides constructs a response datagram known as a DHCP offer and sends it via broadcast to the computer that issued the DHCP discover. This broadcast is sent to UDP port 67 and contains the physi- cal address of the DHCP client. Also contained in the DHCP offer are the physical and IP addresses of the DHCP server, as well as the values for the IP address and subnet mask that are being offered to the DHCP client. From the Library of Athicom Parinayakosol ptg 218 HOUR 12: Automatic Configuration At this point it is possible for the DHCP client to receive several DHCP offers, assuming there are multiple DHCP servers with the capability to offer the DHCP client an IP address. In most cases, the DHCP client accepts the first DHCP offer that arrives. 3. DHCPREQUEST—The client selects an offer and constructs and broadcasts a DHCP request datagram. The DHCP request datagram contains the IP address of the server that issued the offer and the physical address of the DHCP client. The DHCP request performs two basic tasks. First it tells the selected DHCP server that the client requests it to assign the DHCP client an IP address (and other configuration settings). Second, it notifies all other DHCP servers with outstanding offers that their offers were not accepted. 4. DHCPACK—When the DHCP server from which the offer was selected receives the DHCP request datagram, it constructs the final datagram of the lease process. This datagram is known as a DHCP ack (short for acknowledgment). The DHCP ack includes an IP address and subnet mask for the DHCP client. Optionally, the DHCP client is often also configured with IP addresses for the default gateway, several DNS servers, and possibly one or two WINS servers. In addition to IP addresses, the DHCP client can receive other configuration information such as a NetBIOS node type, which can change the order of NetBIOS name resolution. Three other key fields are contained in the DHCP ack, all of which indicate time periods. One field identifies the length of the lease. Two other time fields, known as T1 and T2, are used when the client attempts to renew its lease. 1. “Any DHCP servers out here?” 2. “Here’s my address. Try this address…” 3. “I like that address.” 4. “OK. Keep it temporarily.” DHCP Client DHCP Server FIGURE 12.1 A DHCP server provides the network client with an IP address. From the Library of Athicom Parinayakosol [...]... implement a native IPv6 network at the local level, they might run into problems finding an Internet service provider that offers native IPv6 support Internet IPv6 service is often available through IPv6 tunnel brokers A tunnel broker encapsulates IPv6 packets within an IPv4 tunnel This approach does indeed provide IPv6 connectivity at the end points, but supporting IPv6 through an IPv4 tunnel reduces... technologies This new system eventually crystallized into IP version 6 (IPv6), which is sometimes called IPng for IP next generation The current IPv6 specification is RFC 2 46 0, which appeared in December 1998 (Several other preliminary RFCs set the stage for RFC 2 46 0, and newer RFCs continue to discuss issues relating to IPv6.) The IP address format in IPv6 calls for 128-bit addresses Part of the reason... book will probably be the IPv6 edition, and the topic of IPv6 will inhabit Hour 4 instead of Hour 13 In the meantime, this hour outlines some important IPv6 concepts IPv6 Header Format The IPv6 header format is shown in Figure 13.1 Note that the basic IPv6 header is actually simpler than the corresponding IPv4 header Part of the reason for the header’s simplicity is that detailed information is relegated... The IPv6 addressing system provides a means for accommodating existing IPv4 addresses within the address space The original plan was to map every valid IPv4 address to a 128-bit IPv6 address by simply preceding the original address by 96 zero bits This form, which is known as the IPv4-compatible IPv6 address, was deprecated with RFC 4291 in favor of an alternative technique, known as the IPv4-mapped IPv6... confidentiality Using IPv6’s ESP capabilities, some or all of the data being transmitted can be encrypted Using tunnel-mode ESP, an entire IP datagram is encrypted and placed in an outer, unencrypted datagram In Transport node ESP, authentication data and ESP header information are encrypted From the Library of Athicom Parinayakosol 2 36 HOUR 13: IPv6—The Next Generation IPv6 Addressing IPv6 addresses, like IPv4... in gradually A fullscale retooling of the Internet isn’t going to happen, so engineers designed IPv6 so that it could coexist with IPv4 over a long-term transition From the Library of Athicom Parinayakosol 238 HOUR 13: IPv6—The Next Generation The intention is that an IPv6 protocol stack will operate beside the IPv4 protocol stack in a multiprotocol configuration, just as IPv4 once coexisted with IPX/SPX,... for a way to return to it with TCP/ IP The first step along this path was a concept called Link Local Addressing (IPv4LL) Link Local Addressing has been a part of Apple systems since OS 9, and it has been included in Windows since Windows 98 Microsoft calls the Windows version of IPv4LL Automatic Private IP Addressing (APIPA) If a Windows computer doesn’t have a static IP address and can’t receive a dynamic... writing and simplifying IPv6 address Map existing IPv4 addresses to the IPv6 address space Why a New IP? The IP addressing system described in Hour 4, “The Internet Layer,” has served the Internet community for nearly a generation, and those who developed it are justifiably proud of how far TCP/ IP has come But the Internet community has one big problem: The world might run out of addresses This looming... must provide IPv6 connectivity for Internet-facing servers, and organizations should support internal IPv6 connectivity The draft is set to expire on August 2008; presumably an updated version will be available by the time this book reaches print IPv6 networking is already starting to appear more frequently in OS documentation and training curricula If the transition outlined in the current Internet draft... advanced routing and quality-of-service features built into IPv6 From the Library of Athicom Parinayakosol 232 HOUR 13: IPv6—The Next Generation An Internet draft currently available through the IETF outlines a path to full IPv6 implementation by January 2012 According to the plan, at the end of this transition, Internet providers must offer IPv6 services (and should provide native IPv6 services), . called Link Local Addressing (IPv4LL). Link Local Addressing has been a part of Apple systems since OS 9, and it has been included in Windows since Windows 98. Microsoft calls the Windows version. computer. . If your computer has a domain name, ping your FQDN. . Determine whether IP is configured to use a DNS server. If so, try the following pings: ping www.internic.net ping www.whitehouse.gov . Use. network), static IP addressing has some limitations. The principal shortcomings of static IP addressing are . More configuration—Each client must be configured individually. A change to the IP address