In a TCP/IP environment communication between hosts and servers on the internetwork is transparent.
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This is because each node using the TCP/IP protocol suite has at least one unique 32-bit Internet Protocol (IP) address.
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A router "reads" the destination address in an IP packet and makes a routing decision based on this address.
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All hosts on an internetwork require unique addresses.
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IP addresses can be represented as a group of four decimal numbers (in the range 0 to 255), separated by decimal points.
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This is known as dotted decimal notation.
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They can also be represented in binary or hex.
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For example, 172.16.121.1 is a valid IP address.
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Each IP address identifies a host within the internetwork.
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Part of an IP address identifies a particular network, part identifies a subnet, and part identifies a specific host within that subnetwork.
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Subnet addresses are only present if the network administrator has decided that the network should be divided into subnetworks.
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Each part of an address is represented in binary as an octet, or group of eight bits.
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The minimum value an octet can have is eight zeros equal to zero.
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The maximum value is eight ones binary (11111111), equal to 255 decimal.
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The IP addressing scheme is central to the process of routing IP packets through internetworks.
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IP addresses have two logical parts:

• the network address
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• the host address
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The network address component must be the same for all computers on a given network, and different from that of other networks.
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The node address component must be unique between hosts on a given network, but can be the same between hosts on different networks.
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For example, suppose a network has been allocated the network address 172.16.
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Suppose also that the administrator of this network has assigned the node address 122.204 to a host on this network.
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The complete address of this host is 172.16.122.204.
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This address is unique within the internetwork because only one network can have the address 172.16 and only one host within that network can have the address 122.204.
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<F>There are five classes of IP addresses denoted by the letters A, B, C, D, and E.
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The leftmost (high order) bits indicate the network class.
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The class of address determines which part of the IP address is the network component and which is the node.
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Routers examine the left-most (or most significant) bits of the first octet of the IP address when checking the class of a network address.
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This is called the first octet rule.
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In a class A address (also known as /8), the first octet contains the entire network component.
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The other three octets make up the node address.
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The first bit in the first octet of a class A network address must be set to 0.
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Although it would appear that there are 128 (two to the power of seven) possible class A network addresses, the address 00000000 is not available, so there are, in fact, only 127 such addresses.
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This is further reduced to 126 since network 127.0.0.0 is reserved for loopback purposes.
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The remaining three octets of a class A address are reserved for the node address.
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However, the node addresses 00000000.00000000.00000000 and 11111111.11111111.11111111 are not available in any address class.
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This means that each class A network can support 126 networks corresponding to 16,777,214 nodes (two to the power of twenty-four, minus two).
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A class A address provides for a large amount of node addressing and is therefore capable of supporting very large networks.
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Class A addresses are no longer available as they have all been allocated.
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Class B (also known as /16) network addresses use the first two octets as their network component.
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The first two bits of a class B address are always 10.
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With six bits of the first octet and eight bits in the second octet determining the network address, there are 16,384 (two to the power of fourteen) possible class B network addresses.
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The first octet of a class B address must lie between 128 to 191 (decimal) or 10000000 and 10111111 (binary).
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The remaining two octets of a class B address make up the node address component, this corresponds to 65,534 nodes per network.
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In a class C network address (also known as /24), the first three octets are devoted to the network component.
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The first three bits of a class C address must be 110.
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This means the first octet of class C addresses must lie in the range 192 - 223 (decimal) or 11000000 to 11011111 (binary).
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With five bits of the first octet and eight bits of the second and third octets determining the network address, there are 2,097,152 (two to the power of twenty-one) possible class C addresses.
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The node address component of a class C address is the last octet.
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This corresponds to 254 nodes per network.
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In addition to the addresses assigned for class A, B, and C networks, there are also some special addresses that do not refer to individual networks.
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These addresses are termed class D and class E.
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There are no corresponding network types for these addresses.
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The first octets of class D addresses lie in the range 224 to 239 - they begin with the binary digits 1110.
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Class D addresses are used for multicast packets.
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Many protocols use multicast packets to reach multiple groups of hosts.
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ICMP router discovery is an example of a protocol that uses multicast packets.
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A host can discover the addresses of routers on its local segment by sending a request to 224.0.0.2.
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Any local routers will recognize and respond to this address.
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Addresses in the range 240.0.0.0 to 255.255.255.255 are termed class E addresses.
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The first octets of these addresses begin with the binary digits 1111.
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These addresses are reserved for future additions to the IP addressing scheme.
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A router usually has an interface for each network to which it connects.
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Each router's network interface may be configured with a non-zero IP address and subnet mask.
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Nodes belonging to a network connected to a router may send data to the router interface IP address for that network.
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For example, here Ethernet 0 has an IP address of 172.16.2.1.
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This is the router IP address for any node sending data from network 172.16.0.0.
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This interface will direct data into or out of the 172.16.0.0 network.
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Likewise, Ethernet 1 has an IP address of 10.6.24.2, - it handles traffic into and out of that network.
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In some situations interfaces can have multiple IP addresses.
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Suppose you require more nodes on a network, you can assign a second network address which will yield more IP addresses.
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If you assign another network address, you have to give the Ethernet interface a second IP address, and this is known as a secondary address.
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Communication between the two networks is handled by the router, but they cannot talk to each other even though they are on the same wire.
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An IP address with a value of zero (for example, the class B address 172.16.0.0) is the general network address.
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The network address without specified node addresses was used in early implementations of TCP/IP as a broadcast address.
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An IP address of ones (or 255.255.255.255 decimal) refers to all networks or all hosts.
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It is reserved for IP broadcasts to a network.
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The default IP address for a router is comprised of zeros.
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Routing tables contain entries for network or "wire" addresses including subnets, if they exist.
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They do not usually contain host address information.
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However, they do contain hardware addresses in order to send a packet to a host on a network.
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The hardware addresses are obtained using the ARP protocol.
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Often data is sent over the Internet based on the name of a person or organization.
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For network users it's more convenient to associate a name with each IP address.
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For example, the IP address 172.16.121.1 may be associated with a company name.
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A typical class B IP address comprises a network address and a node address.
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The network is the first two octets of the IP number (172.16).
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The third and fourth octets identify the node address (121.1).
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If a subnet is specified, this uses some of the node addressing space.
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How much node addressing space can be used depends on the network class.
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If names are used instead of addresses, the names must be translated to IP addresses before the data can be delivered.
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A company with a class B network has an IP address, of which the first two octets make up the network address.
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Most applications will accept either an IP address (172.16.121.1) or a host name as an argument.
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In this case, both ^RTelnet 172.16.121.1^r and ^RTelnet^r ^Bhostname^b will open a remote terminal session with the specified host.
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While host names are convenient for users, they pose a problem for internetwork routers, which deal only with IP addresses.
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When a user specifies a host name in a command, the name must be translated to the appropriate IP address.
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There are three commonly used methods of mapping host names to IP addresses:

• host tables and network tables
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• Domain Name System (DNS)
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• Network Information Service (NIS)
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A host table is an ASCII file that contains a list of the names of remote hosts known to the local computer.
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Each name is stored together with the appropriate IP address.
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Host tables store frequently used IP addresses, but are not suitable for very large networks such as the Internet.
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On UNIX systems, the host table is usually stored in a file called /etc/hosts.
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A network table is similar to a host table.
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However, it stores network names, addresses, and aliases of known networks.
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Addresses in the host table always have four numbers to uniquely identify a network and a node.
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Addresses in the network table do not refer to individual nodes, so they have only one or two numbers (depending on the network addressing type).
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The network table is usually stored as /etc/networks on UNIX systems.
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The Internet has far too many computers attached to it for a a single host file to be a viable option.
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The Domain Name System (DNS) is widely used on the Internet to translate host names to IP addresses.
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DNS divides the Internet into domains which are arranged in a hierarchical manner, starting with a root domain.
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Each domain must contain a ^Rname server^r - a computer that translates names within that domain.
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Name servers contain host address databases for all hosts within their own domains.
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A DNS server can resolve any IP address on the Internet by referring requests to other DNS servers.
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Here are some top-level domains on the Internet:

• MIL for military sites
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• EDU for colleges and universities
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• COM for business
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• GOV for government organizations
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• ORG for other organizations
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• NET for gateways and hosts
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There are separate top-levels domains for most countries - for example, DE for Germany, UK for the United Kingdom, US for the United States, and so on.
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Network Information Service (NIS) is another type of naming service often used on IP internetworks.
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Unlike DNS, NIS does not provide name translation for the entire Internet.
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Instead, it is intended to provide a convenient way for network administrators to maintain addresses for a group of computers in a single database.
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NIS groups hosts into domains, but these are not equivalent to DNS domains.
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An organization can define its own IP addressing scheme for internal purposes, but it is not valid for Internet access.
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To obtain Internet access, an IP address must be obtained from a registration authority.
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Network Information Centers (NIC) around the world handle the assigning of Internet addresses.
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In America, IP numbers are assigned by the InterNIC Internet registration service, which is operated by Network Solutions, Inc.
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It maintains a library of templates for registering IP numbers and domain names.
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You can register a domain name by downloading the registration template, filling it out, and e-mailing it back to InterNIC.
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In Europe registration is performed by RIPE.
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Many Internet service providers can supply domain names with the approval of the NIC.
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InterNIC can help you to determine how many Internet addresses you need based on

• how many individual
  network segments you have
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• how many systems on each
  segment are to be connected
  to the Internet
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When you know how many IP addresses your organization will need, you will have to configure a DNS server to maintain the addresses and the domain names.
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When an organization registers a domain name, the registration authority enters that name into the global domain name system.
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Any Internet user can then use the domain name to communicate with that organization over the Internet.
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In most cases when you transmit data, the router finds the destination's IP address in the host or network tables.
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Suppose you want to send data using a host name unknown to your router, what happens?
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Provided there is no entry in the routers host table and the host name is registered, the router sends a DNS request for an IP address corresponding to the host name.
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DNS refers the request to its domain servers until a matching IP address is found.
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A DNS reply returns the IP address to the router.
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The IP address indicates the location of the host on the Internet, and the router uses it to choose a path for the data.
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The location of the destination host will determine the path the data follows through the Internet.
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The data is transmitted to the next router on its path, and in turn this router will select a path to an appropriate router.
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This continues until the data reaches the destination host.
