Chapter 4 teaches the basics of how IP addressing works. The objectives important to this chapter are on page 4-1:
Concepts:The purpose of an IP address is to identify each unique node on a network. On an IP network, each device is known as a host, and every host must have an address. The addresses we discuss first are actually IP version 4 addresses. (IPv6 addresses will be 16 bytes, or 128 bits long.) They are numeric addresses, stored as four bytes, which is equal to 32 bits. When we write these addresses, we usually place dots between the bytes, but you must understand that the dots do not exist in the addresses as used. The text refers to this notation as dotted octet. (Each byte is called an octet because it has eight bits.) Since the IP protocol stack was invented with networking in mind, IP addresses contain two parts: one to identify the address of the network a host is on, and the other part to identify the host itself. Every network is assigned an address which could be one, two, or three bytes, depending on the class of the network (A, B, or C). The remaining byte or bytes are typically used for hosts on networks. (Variations will be discussed in the next two chapters.) Each byte in an IP address will be a number in the range 0 through 255, expressed in base 10 (decimal notation). To pass the certification test for this course, you will need to be able to convert decimal notation to binary notation and vice versa, by hand. No calculator conversion is currently allowed. Appendix F of your text provides a conversion chart, but you will need to know a conversion method to pass the test. Page 4-5 shows a good chart for the five address classes you need to know. The first three classes can be described by the number of bytes assigned to the network portion of their addresses:
Class D and E addresses use portions of the fourth byte as well for network addressing. You may wish to know that only class A, B, and C addresses are in general use. Class D addresses are for multicasting, and class E addresses are for experimental use. A better way to recognize the class of a given address is shown on page 4-6. The five classes of addresses are defined as limited to specific ranges within the first byte. The numeric ranges are hard to remember until you see a chart like that on page 4-6 that explains the ranges have to do with the binary version of the first octet. Some basic training in binary notation: a byte has eight bits. Each bit can be a digit in a binary number. Since we can only use 1s and 0s in binary notation, we either have (1) or don't have (0) the number of units represented by a position in the binary number.
Reading from left to right, if the first bit (position 7, above) of the first octet is a zero, that octet must represent a number less than 128. This defines a class A address: the first octet must be 127 or less. Consider it this way:
So, if you can convert the first octet of an address to binary notation, you can tell the address class by the position of the first 0 in it. The discussion of the address classes continues to tell us that most possible network addresses (not host addresses) have been assigned already. Page 4-8 has a table with some specific addresses that have special meanings:
Page 4-11 continues with the discussion of how you get an address license for your network. First, you should understand why. IP addressing is used on the Internet, which has become the common network of the world. To use the Internet, you need a unique IP address, else your requests would not be identified as yours. Of course, if your network will never connect to the Internet, or with other networks using IP addresses, you wouldn't need registered addresses. InterNIC, also called Network Solutions, Inc., is mentioned on page 4-11 as a source of addresses for Internet Service Providers (ISPs), if you are in the United States. These ISPs often assign addresses to smaller ISPs, who eventually assign them to applicants like companies (for networks) and individuals (for dial-up accounts). This duty is now being handled by the American Registry for Internet Numbers (ARIN) for U.S. customers, by Réseaux IP Européens (RIPE) for European customers, and by Asia Pacific Network Information Center (APNIC), for customers in Asia and the Pacific rim. The text also states that InterNIC and ARIN administer North America, South America, South Africa, and the Caribbean. (Two other designees exist within ARIN: RNP (Brazilian Registry) and NIC-Mexico (Mexican Registry).) Page 4-13 briefly speaks to the issue of a current shortage of IP addresses. Returning unused addresses is a requirement. It is anticipated that IPv6 (also called IPng, for IP Next Generation, here) will resolve the current shortage. A registered domain name is a name that is unique in the DNS hierarchy. The basic idea is illustrated on page 4-14. The naming system is divided into domains, each of which has a server assigned to it as the authority at its level. There is a server, for example, that has the duty of knowing every name assigned in the COM domain, the domain for commercial entities in the United States. A table of commonly used domains appears on page 4-15. Be aware that this table applies mainly to the United States (and some names registered in Canada). The next tables on 4-15 and 4-16 illustrate that the top level domains in other countries are named for the country itself, using two letter abbreviations. (e.g. au for Australia, ca for Canada, de for Germany) Several hosts can be used within an assigned domain name. For example, Novell has registered the domain name NOVELL.COM, and within in there are several hosts you may wish to browse: Support.Novell.Com, and Education.Novell.Com are popular with students. Notice that I used an uncommon capitalization in the last sentence. Capitalization is unimportant to domain names. Length is important: the entire name cannot be more than 255 characters long. Each label (support, Novell and com are labels) can be up to 63 characters long. To apply for a domain name, go to your ISP, or to the manager of the top-level domain where you want the domain (this is generally for non-U.S. domains). Page 4-19 discusses assigning addresses to hosts within your network. The first example shows that host addresses should follow the network address scheme you are assigned. If your network address was 150.60.0.0, you would have a class B address, and all host IP addresses should begin with 150.60. (The next two chapters will show you variations on this.) If you do not have an assigned address, you could use any address scheme you wanted as long as you did not attach to the Internet. However, in reality, everyone wants or needs the Internet, so you should use a private address scheme, as shown in the chart on page 4-19.
Any address beginning with a 10, for example, is assumed to be a private address. This is the format used in a network I have some experience with. To access the Internet, traffic passes through a server that acts as a proxy, providing a shared connection with a registered address. Page 4-20 describes two methods for translation of host names to IP addresses. One is Domain Name Service (DNS), and the other is the use of host tables. A host table is a list, divided into three columns: IP address, official host name, and aliases that are also allowed for this host. Each line in the table describes one IP address and the host names that can be used for it by machines that read this table. This is a fine system for small networks in which changes do not happen often. Each device on the network has to know where the host table is stored, and must read it to make use of host names. On a Novell system, we are told that it is usually stored at SYS:ETC\HOSTS, and on a UNIX system it is usually /etc/hosts (note that neither example uses a file extension). Regarding syntax for the host file itself, it must be an ASCII file, in which the "#" symbol marks the beginning of a remark or comment. (It has another use as well, but that can wait.) Using DNS is different. The information needed to find a host on the Internet is too complex and too fluid to assume it is saved on any single server. Domains are divided into zones (subdomains). The text informs us the Novell.Com represents a zone that Novell (the company) administers. This zone is subdivided into other, smaller zones that are administered by divisions of the company. The graphic on page 4-22 illustrates the idea that the DNS hierarchy is meant to be subdivided. A DNS server consults higher and higher level authorities, which consult zone authorities under them to resolve a DNS request. The machine asking the server for the translation of a host name to an IP address is called a DNS resolver. Two other zone types are listed on page 4-23:
Page 4-24 discusses Master Name Servers. Each DNS zone must have one Master Name Server which holds the authoritative database for its zone. This database may be copied to replica (or secondary) DNS servers, but the authoritative database is the only one in which changes and updates may be made. Other features listed on page 4-24:
The Master Name Server for a zone is not expected to perform all DNS service in the zone. Replica name servers are located at convenient locations from which to respond to user requests. They periodically check the serial number of their copy of the database against the serial number of the copy on the Master Name Server (which it increments when changes are made). If there is a difference, the replica name server requests a zone transfer, which is a request for a new copy of the authoritative database. Note that a replica name server does not have to be located in the zone it holds a replica of. Page 4-26 defines the term DNS resolver. As explained above, this term applies to any host that requests the translation of a DNS name to an address. The address the host needs may not be in any database stored in its zone. Master Name Servers pass the request up and down the tree until the answer is found. In the example in the text, the host wants the address of FTP.NOVEL.DE. The servers in the resolver's zone pass the request up to the Master Name Server for the DE (Germany) zone. It sends the request to the NOVELL Master Name Server in the DE zone. The NOVELL server will know the address of the FTP server in its zone, so the information is found and the reply is passed back to the resolver via its original Master Name Server, which will store the information as well. |