Network Protocol:
TCP/IP

What is protocol?

Networks need a set of rules and procedures to govern the transmission of data. These rules together form a protocol. A typical protocol includes the following specification:

 

• the type of error checking to be used
• data compression method, if any
• how the sending device will indicate that it has finished sending a message
• how the receiving device will indicate that it has received a message

Different network products have different protocols such as AppleTalk in Macintosh networks, DLC protocol in HP network printers, NetBEUI in Windows networks, IPX in Novell Netware networks, TCP/IP in UNIX and the Internet. Nonetheless, TCP/IP is gaining more and more popularity and therefore only TCP/IP will be introduced here.

Different protcols are still available but usually TCP/IP alone is sufficient for networking. You should install what is/are needed rather than accepting the default. More installed protocols will slow down the performance of your computer, and even the network.

TCP/IP was developed in the 1970s by and for the United States Department of Defense (DoD) as well as several research organizations. TCP/IP is not just a protocol. Rather it is a protocol suite, which consists of Transmission Control Protocol, User Datagram Protocol (UDP), and Internet Protocol. Only TCP and IP will be discussed here. TCP divides data into smaller segments for transmission as well as provides error checking and flow control. IP performs the function of addressing and selecting the routes of connection.

The following are the Open System Interconnection (OSI) model and the DoD's TCP/IP model, which are conceptual frameworks of how networks function. If you are not a network administrator, the detail of these models are irrelevant to you. The most important points are:

• TCP and IP operate at different layers of network models.
• Data are formatted differently when they are passed from one network layer to another.
• Computers communicate with each other by via ports using TCP and IP addresses.

IP Address

When you mail a letter in the post office, you need to write down the address on the envelope. By the same token, transferring data among computing devices requires addressing. IP handles addressing by assigning an IP number to every computing device on the TCP/IP network. I say computing device rather than computer because a printer could have an IP address if TCP/IP printing is used.

Let's use mailing address as an analogy again. A mailing address consists of different levels such as country, state, city, street, and apartment number. Similarly, an IP address is composed of four chunks of 8-bit binary numbers such as 129.219.199.15, which represent different levels of the address. For easier identification, these binary numbers are translated into decimal numbers that we use everyday.

The metaphor of physical address falls apart when it involves the concept of logical addressing. In physical addressing, apartment 1 and apartment 2 should be physically next to each other. However, IP addressing groups computing devices logically according to administrative needs. Two computers which carry IP addresses as 120.219.199.10 and 129.219.199.11 may have no geographical proximity at all.

 

Subnet Mask

A state is divided into several counties. Similarly, a network is spilt into many sub-networks. The Subnet Mask indicates how many subnetworks are located in this network and determine whether routing is necessary.

 

Router/Gateway

When computing devices are located in the same subnet, they can connect to each other directly without an intermediate station.

But when they do not belong to the same subnet, a router or a gateway is needed to connect them.

 

In Mac networking the term "router" is used while in Windows networking the term "gateway" is used. Today these two terms are almost inter-changeable, but indeed they are slightly different. A router passes information among networks with the same protocol whereas a gateway connects different networks and translates between protocols. *

 

It is note-worthy that the principle of routing, to certain extent, goes against common sense. For instance, if my computer is located in Phoenix, Arizona and I want to access a Web server in Tempe, which is less than 10 miles away from Phoenix, my common sense will tell me that the best route is to go straight from Phoneix to Tempe. Actually, routers may take several criteria other than distance into account while determining the best route (e.g. load, reliability...etc). The best route, as shown in the following figure, may be Phoenix--Denver--Dallas--Los Angeles--San Jose--Tempe.

 

Domain Name Services

In order to pass information to the correct destination, a router or a gateway must be able to identify the addresses of computing devices. To achieve this, it looks up the address book in a Domain Name Services (DNS) Server. A DNS server carries a global database listing names and IP numbers of every TCP/IP computing device. Usually in addition to the IP number, some computing devices also has a domain name. For example, let's say the IP address of a server is 129.218.11.111 and the name is "www.alexyu.com" When you enter the domain name in a browser, DNS translates the name into IP number so that a router knows which computer is the destination.

 

This look up procedure slows down the network traffic slightly. It is faster to skip DNS and address a computer with the IP number. Further, if the DNS server is down, you will not be able to perform networking by using names. In this case, you should use IP numbers instead. Actually it happens occasionally. When the DNS servers are not reachable, the user could not do anything with the Internet as usual even though everything else is working properly. Unfortunately, when you use some tool to test the network connection, it may show that no problem is found. The following screen shot shows a typical example. The test result returned by the software application shows that "good connection to your ISP and Internet," but indeed DNS is down.

 

 

If an IP number alone is sufficient and even more efficient, then why do people use domain names? Because it is easier to remember "alexyu.com" than recalling "129.218.11.111." However, one cannot just make up a domain name. Like IP numbers, domain names must be unique and require registration. The mapping between the two is stored in global databases. To register a domain name, visit an authorized registration site such as networksolutions.org or register.com

Dynamic DNS

It is important to note that unlike WINS, which will be introduced in the following section, most DNS servers are static i.e. they do not update themselves when a new IP and hostname are assigned to a node. Nevertheless, a new standard called Dynamic DNS is on the horizon. As its name implies, dynamic DNS is able to receive registration of new IPs and hostnames automatically. Although Windows 2000 has a feature to register the machine to dynamic DNS servers, it may not work when your centralized DNS server is static. Leaving the registration on would create unnecessary traffic while your computer keeps sending registration to a static DNS server.

 

Windows Internet Naming Service (WINS)

Like DNS, WINS also resolves names and IP addresses except that the look up by WINS is specific for Windows computers. DNS keeps track of all TCP/IP addresses on earth while WINS usually keep records of Windows computer addresses in the LAN, MAN, or WAN only. In addition, unlike DNS in which records are static, WINS is dynamic i.e. A WINS server frequently scans Windows computers currently available on the network for the assigned IP addresses and automatically update these data.

Notes: Windows 2000 uses DNS to replace WINS. If you have a pure Windows 2000 network, WINS is not neccessary.

 

NS Lookup

You may wonder how you can get the above information (IP, DNS, WINS...etc.). Ask your network administrator, of course. But what if he is not around? A quicker way is to use a software package for Name Services Lookup (NS Lookup), which is available in both PC and Mac platforms. The following figure shows an example of Mac NS Lookup. When you enter a hostname or a IP address into the search field, NS LookUp can consult available DNS servers and return the translated IP number or the hostname. When you press the submit button, it can reveal more information such as the IP numbers of the router, DNS servers, and WIN servers associated with the computer. This tool is handy when you forgot this information. For instance, once my coworker and I wiped out a server's hard drive and did a clean installation. However, he forgot to write down the IP numbers of the server, the router, the DNS servers, and the WINS servers. He could remember only the hostname of the server. As you expect, NS LookUp rescued us!

 

IPConfig

In addition to NSLookUp, another handy TCP/IP-related tool is IPConfig (for Windows NT and Windows 2000). When you cannot connect to the Internet, don't panic. First, open a command-line session (a DOS-like window) by typing cmd in Run. Then type IPCONFIG in the command session window. As the name implies, IPConfig shows you the TCP/IP configuration of your computer. From there you can see whether everything is configured properly. To reveal more details, type IPCONFIG/ALL. If your computer's OS is Windows 9x, please use WINIPCFG.

Socket/Port

The preceding section explains that TCP/IP is a protocol suite, IP and subnet mask are parts of the addressing scheme, and DNS and WINS are tools for address/name resolution. However, two computers need more than a protocol suite and addresses to establish a communication channel. They need a socket: Socket = port + IP address + Transport layer protocol

Ports in a computer are like "airports." They are points of entry for data transportation. For example, a Web server opens Port 80 for users to browse webpages; a FTP server opens Port 21 for users to upload and download files. Usually ports are transparent to users. Nevertheless, you may want to know more about it if you are curious about how a hacker intrude a computer system. First, the hacker must know the IP address of the target computer. Second, he/she uses port scanner software to look for all vulnerable ports of a computer. When she/he finds an open port and sends a compatible protocol, a socket is established and the computer is hacked!

The first line of defense is NOT to expose your IP address. If your computer is not a public server but you still need to provide some network services to your coworkers, you can change the port number from the default setting to something else. For example, the default port number of a FTP server is 21. If you change it to 9999, then your coworker must specify the exact port number in order to access the server.

Another way to secure your computer is to disable unneccessary services so that unused ports are shut down. Also, you can install a Firewall to block malicious port scanning. A firewall is a software- or hardware-based product that prevent unauthorized users from gaining access to a computer network or that monitor transfers of information to and from the network.

Note

* Do not perceive that a router is only for big networks such as inter-networks across continents. You can have a router at your home. A router, by definition, is a device to connect two distinct networks. At home you can use a Cable modem/DSL router to connect your local area network (LAN) inside your house with the wide area network (the internet). The following figure shows the configuration of a "home" router. As you see, the configuration includes information of LAN IP and WAN IP. Basically, the WAN IP is assigned by your internet service provider and the LAN IP is assigned by the router.

 

In addition, a router is not necessarily a specialized device. Any computer that has multiple network interface cards can be configured to be a router. Both Novell Netware and Windows 2000 Server has built-in IP routing capability. The following figure is a screen shot of the IP routing configuration in Windows Server.

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