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Computer Networking with TCP/IP Automatic addressing on TCP/IP networks Manually assigning IP addresses to individual computers on a network can quickly become a hassle. The answer is to use DHCP and WINS servers, so that IP addresses are handed out automatically each time a machine boots up. By GREG SWAIN & BOB DYBALL Non-routable networking protocols such as NetBeui are fine for small networks, where you only have a few PCs hooked up to one or more hubs and no routers are involved. In fact, NetBeui is one of the most effective protocols for up to about 20 people. It’s fast and easy to configure because you only have to provide each computer with a name – no network or logical addresses are required. However, it’s not uncommon for a network to be segmented using switch­ es and routers. This can be done for any number of reasons but usually it’s done to filter unnecessary network traffic from certain sections, to increase network performance. Bridges and routers are also often used to couple large networks in adjacent buildings together. And that’s where you run into Fig.1(a): here’s how to manually set up an IP address under Windows 98. Each computer on the network must be given a unique IP address, while the Subnet Mask should be the same for all machines. 4  Silicon Chip problems with NetBeui. Because it’s non-routable, network segments on either side of a router will not be able to “talk” to each other. To overcome this problem, a rout­able protocol such as TCP/IP must be used. Although it might initially appear rather mysterious, TCP/IP isn’t all that hard to get going – at least, in basic form. The first step is to install the TCP/IP protocol (if it hasn’t already been installed) and you do that via the Network utility in Control Panel. The procedure is as follows: load the utility, click the Add button, select Protocol, click Add again, select Microsoft, select TCP/IP and click OK – see Fig.1 You can now set up the IP address for the computer by selecting the TCP/ IP protocol (for the network card, not for a dial-up adapter) and clicking Fig.1(b): this screen grab shows how to manually enter IP addresses under Windows NT. The “Default Gateway” is the router address if it exists (otherwise leave it blank). the Properties button. Note that the IP address set here is “bound” to the network card and is non-routable as far as the Internet is concerned. Basically, there are three groups of IP numbers to choose from for private LANs. These address blocks are as follows: 10.0.0.0  to  10.255.255.255 172.16.0.0  to  172.31.255.255 192.168.0.0  to  192.168.255.255 When you manually assign IP addresses, each machine must be given a unique address but the Subnet Mask must be the same if you want the machines to “see” each other on the network. For most networks, a Subnet Mask of 255.255.255.0 will do just fine but note that, if you use this, all your IP addresses must use the same first three groups of numbers; ie, only the last number can be varied. Note also that it’s not usual to end an IP address in 0 or 255. So, for example, if you use a Subnet Mask of 255.255.255.0, you could use IP addresses ranging from 192.168.1.1 to 192.168.1.254. Each machine must also be given a unique NetBios name (or Computer name), as shown in Fig.2. The Work­ group name should be the same for all machines on the network.. Don’t forget to set up file and printer sharing on each machine, so that you can access the desired resources. The LMHOSTS file PCs on a network running TCP/IP need some mechanism to link the IP address of each machine to its NetBios name. This is known as “name resolution” and is necessary to allow computers on the network to communicate with each other. The reason for this is that users generally attempt to communicate with another computer by selecting its NetBios name. However, before communication can take place, the NetBios name must first be translated into that computer’s IP address. Two common methods for achieving this are “broadcast name resolution” and using an lmhosts file. The first method involves broadcasting a query over the network and asking a named computer to respond with its IP address. This is quite satisfactory if there are only a few machines on the network but generates a lot of unnecessary traffic on larger networks. An lmhosts file eliminates the need Fig.2: each machine on the network must be given a unique name, while the Workgroup name must be the same if you want the machines on a LAN segment to “see” each other. Fig.3: Example lmhosts File # IP Address Computer name # 192.168.1.1 lister 192.168.1.20 rimmer 192.168.1.40 starbug 192.168.1.80 holly and don’t do anything. In practice, all you have to do is create the lmhosts file using a text editor and copy it to all the machines on the network. On Windows 9x systems, this file must be placed in the c:\ windows folder (assuming that that’s where Windows is installed), while in Windows NT the file must be in c:\ winnt\system32\drivers\etc. If you are using Windows 95/98 you must also select the “Enable DNS” option in the TCP/IP properties dialog box (see Fig.4) and enter in a host name (this is usually the same as the computer name). If you don’t do this, the system will automatically resort to broadcast name resolution. Similarly, for Windows NT, you have to select “Enable Lmhosts Lookup”. This is done at the WINS Address tab in the TCP/IP Properties dialog box – see Fig.5. Note that each machine on the network must have the same lmhosts file, otherwise you will run into problems. IP addressing with DHCP for these query broadcasts. This is a simple text file that lists the IP address of each machine on the network and its corresponding NetBios name. Fig.3 shows a sample lmhosts file. Its format is fairly straightforward. The lines starting with “#” are comments Manually assigning IP addresses and maintaining an lmhosts file is fine if you only have a few computers on the network. However, it soon becomes unwieldy if the network is constantly changing or if you have more than about 15 computers to administer. Each time a machine is added to the network, for example, the lmhosts file must be altered and copied to each existing machine. As you can imagine, it’s all too easy to assign a new machine with a dupli- Fig.4: if lmhosts is used, you must also select the “Enable DNS” option in the TCP/IP properties dialog box of a Windows 95/98 client computer, Fig.5: a Windows NT client computer must have “Enable Lmhosts Lookup” selected if you wish to use lmhosts for name resolution. September 1999  5 Fig.6(a): setting up the Scope Properties for a DHCP server. In this example, the server can hand out IP addresses to client computers anywhere in the range from 192.168.1.10 to 192.168.1.200. The lease period is set to one day. cate IP address or to neglect updating the lmhosts file on some machines. In either case, the network will no longer work correctly. Often too, users want to take a laptop computer from one location to another and plug it into the network but this won’t work with static IP addresses if the lmhosts files and/or subnet masks are different. The solution is to use DHCP, or the Dynamic Host Configuration Protocol. This service, which runs on Windows NT Server, is one of the most convenient ways of handing out IP addresses. It initially requires more work because we have to set up a DHCP server but after that, it’s all plain sailing. When you set up the DHCP service, it’s given a range of IP address that can be handed out to client computers. The client computers then request an IP address each time they boot and this is automatically allocated by the DHCP server. Because the server keeps a record of which IP addresses have been allocated to clients, there is no chance of duplication. When a client computer subsequently shuts down, its IP address is returned to the pool of numbers back at the DHCP server for later reuse. This means that there’s no guarantee you’ll be given the same IP address each time your machine boots up but at least you will be the only one with the IP address that is assigned. This scheme has several other advantages. First, you don’t have to manually set a static IP address for each machine on the network. Second, you don’t have to maintain the lmhosts files across the network. And third, it makes it very easy to move a computer from one part of a network to another since it will ask for and be automatically assigned an IP address when it boots. OK, so that’s it in a nutshell. In practice, it’s a bit more complicated than that (it always is). Each time, the DHCP server issues an IP address, it does so for a set “lease” period. This lease period is configured at the server and can last for anything up to several days. During the lease period, the client computer contacts the DHCP server and requests that the lease be extended. If everything is OK, the server then renews the lease on the same IP address and restarts the clock. So what happens if the client computer is shut down and then rebooted during the lease period? In that case, the client remembers that the previous lease hadn’t expired and so it asks for the same IP address again. Because an IP address is reserved for a particular client for the duration of the lease, the DHCP server complies with the request and re-issues the address with the lease starting over again. If the lease expires, a new IP address is issued to the client the next time it boots up. Similarly, if the DHCP server is re­config­ured during the lease, the client’s request for an extension will be rejected and a new address will be issued. Setting up a DHCP server We won’t go into all the details of setting up a DHCP server here, although the procedure is fairly straightforward for a simple network. Basically, you need Windows NT Server or a Linux server to set up the service. This server must be given a static IP address (eg, 192.168.1.1). If you don’t have either of these operating systems, there are several shareware programs that let you set up a Windows 95/98 system as a DHCP server – see panel. In Windows NT Server, you install DHCP by first loading the Network utility in Control Panel, then clicking the Services tab, clicking the Add button and selecting “Microsoft DHCP Server” from the list and clicking OK. Fig.6(b): the DHCP Options dialog boxes let you specify the IP addresses of any other devices on the network, such as routers, WINS servers and DNS servers. In this example, we’ve added a router and specified its IP address as 192.168.1.2. 6  Silicon Chip Fig.7: a Windows 98 system is set up as a DHCP client as shown here. By selecting the option shown, the client will obtain a unique IP address from the DHCP server each time it boots. The installation routine creates a new shortcut labelled “DHCP Manager” in the Administrative Tools folder of the Start menu. Once the DHCP service has been installed, you then specify the Scope Properties. This sets the range of IP addresses that the server can hand out to clients for a particular subnet. Fig.6(a) shows the Scope Properties dialog box from the DHCP Manager on Windows NT Server system. In this case, the range of IP addresses that can be passed out to client computers starts at 192.168.1.10 and ends at 192.168.1.200. The lease duration has been set to one day. Other dialog boxes let you specify the IP addresses of other devices on the network, such as routers or WINS or DNS servers – see Fig6(b). By the way, if network reliability is critical, you need to install more than one DHCP server so that the network will still function if a server fails. In that case, you configure each server with a segment of the range of allowable IP addresses. These segments must not overlap, otherwise the system could issue the same IP address to more than one computer. DHCP client configuration Unlike the server side of things, Windows 95/98 and Windows NT Workstation computers can all be set up as DHCP clients without the need for additional software. Once set up, they will request an IP address from a Fig.8: configuring a DHCP client on a Windows NT system. The gateway address corresponds to a router (if it exists). DHCP server each time they boot. Fig.7 shows how a Windows 98 system is configured, while Fig.8 shows the settings for a Windows NT DHCP client. You get to the Windows 95/98 TCP/IP Properties dialog box by first double-clicking the Network icon in Control Panel, then double-clicking the TCP/IP entry for the network card. Make sure that the “Obtain an IP address automatically” option is selected if you want the machine to request IP addresses from the DHCP server. A Windows NT system is set up in similar fashion. Windows Internet Name Service (WINS) Often used in conjunction with DHCP, WINS is a dynamic database that’s used to translate NetBios names into IP addresses. Without WINS, name resolution takes place either by: (1) broadcasting a query over the network and asking a named computer to respond with its IP address; or (2) checking a local lmhosts file (if one is present). As mentioned previously, the first meth­ od can create a lot of unnecessary traffic and in any case, is limited to a local network segment. The second has all the administrative problems discussed previously. As with DHCP, WINS is provided with Windows NT Server. After set-up, client computers contact the WINS server each time they boot and register their name and allocated IP address. This means that a WINS serv- Getting It Together: The Software Required Windows NT Server will allow you to run any or all of these services; ie, DHCP, WINS and DNS. If you don’t have NT Server, consider Linux as an alternative, as you can run DHCP and DNS with the standard Linux or use WINS under Samba server. Linux is not yet for the faint-hearted, though. Be prepared for a steep learning curve and lots of manual text-mode configuration files. Windows 95/98 users should check out some of the shareware sites on the Internet for suitable software. These sites include www.download.com, www.winfiles.com and www.tucows.com One popular product is WinGate, which runs under Windows 95/98/NT and provides DHCP and DNS server capability. WinGate can also function as a firewall and as a proxy server, to allow multiple users on a network to connect to the Internet via a single modem. To find out more about WinGate, point your web browser to www.jantek.com.au September 1999  7 must select “Enable WINS Resolution” and manually enter the IP address(es) of the WINS server(s) on each client computer. Fig.10 shows how to do this. As with DHCP, it pays to have a more than one WINS server to ensure network reliability. Domain Name Server (DNS) Fig.9: here’s how to set up the WINS configuration on a Windows 95/98 client computer if you have a DHCP server. er automatically updates its database whenever computers are added to or removed from the network. When a client computer needs to resolve a NetBios name, it contacts the WINS server, which then hands out the IP address for that name. In effect, WINS is the dynamic equivalent of an lmhosts file. Because a client computer always attempts to contact a WINS server first for name resolution, the need for query broadcasts or lmhosts file checking is eliminated. Setting up WINS WINS is installed on an NT Server machine in exactly the same way as DHCP. After that, it’s a matter of config­ uring each of the client computers to use WINS for name resolution. If you’re also using DHCP, this can be used to supply the primary (and secondary, if it exists) WINS server address to client computers that ask the DHCP server for an IP address. This saves you from having to type in the WINS server address(es) on each of the client computers. Fig.9 shows how to set up the WINS Configuration on a Windows 98 client Fig.10: if you don’t have a DHCP server, you will have to enter the IP address(es) of the WINS server(s) on each client computer, as shown here. computer if you have a DHCP server. It’s just a matter of selecting the “Use DHCP for WINS Resolution” option. Of course, you must also use the DHCP Manager to supply IP addresses to the WINS servers and to hand these out to the clients. If you don’t have DHCP, then you Fig.11: Example hosts File For Windows 98/NT                127.0.0.1 192.168.1.1 192.168.1.20 192.168.1.40 192.168.1.80 8  Silicon Chip localhost lister.reddwarf.home rimmer.reddwarf.home starbug.reddwarf.home holly.reddwarf.home Oh, no! – not another naming system! Well, yes but this one’s somewhat different from WINS because it allows your system to look up, or “resolve”, a domain name (eg, www. microsoft.com) and translate this to an IP address. Currently, this would give you 207.46.130.14 as the IP address for Microsoft’s web site. In practice, this means that a web site on the Internet can be given a friendly address and you don’t have to worry about typing in its IP address. Instead, a Domain Name Server (DNS) looks up the IP address for you. You don’t have to worry about installing a DNS if you only wish to access the Internet, since this will be taken care of by your Internet Service Provider (ISP). However, you may want to set up a DNS if you wish to run a private intranet. Alternatively, if the network is only small and you are using static IP addresses, you can resolve domain names using a hosts file. This works in a similar manner to an lmhosts file (which resolves computer or NetBios names), except that you list the host names next to the IP address. Fig.11 shows a sample hosts file. Note that, as with lmhosts, you must select the Enable DNS option in the TCP/IP Properties dialog box for Windows 95/98. A similar situation applies to Windows NT. By the way, Windows NT server’s implementation of DNS allows direct look-up of the NetBios names from WINS. If this is enabled, it can make quite a powerful system whereby users are able to find others either by their NetBios name (eg, “Rimmer”) or by their Fully Qualified Domain Name (eg, www.siliconchip.com.au). Finally, this article should only be considered as an introduction to DHCP, WINS and DNS. TCP\IP addressing is really quite a complicated subject and you can buy complete textbooks on this topic. And, of course, it all becomes much more complicated when you throw in a SC dial-up adapter.