Posted by arlene

Although most people think of IP as the transport protocol used by higher-level protocols, one of its more important functions is to provide the address space used by the TCP/IP suite. The difficulty of having to create a routing table that consists of hundreds of millions of actual hardware addresses, providing for no built-in organization capability.

IP addresses are used for just this purpose: to provide a hierarchical address space for networks. Each network adapter has a hard-coded network address that is 48 bits long. When data packets are sent out on the wire of the local area network (LAN) segment, this MAC address is used for the source and destination addresses that are embedded in the Ethernet frame, which encapsulates the actual IP packet. After an IP packet reaches the destination network, the router sends the packet out onto the network segment that contains the destination. The MAC address is used from there on to deliver the data. On a LAN segment, MAC addresses can be used efficiently because most LAN segments consist of just a few hundred or a few thousand host computers. This number of addresses can easily be stored in network devices, such as bridges or switches.

IP Addresses Make Routing Possible

Because the IP address is composed of two components, the network address and the host computer address, it is a simple matter to construct routers that use the network portion of the address to route packets to their destination networks. After the packet has arrived at a router on the destination network, the host portion of the address is used to locate the destination computer. Without the capability to designate a network address, as well as a host address, the hierarchical address space could not exist, and routing would require routing tables that literally would have to store every address of every computer or device on the network. In such a scenario the IP address would have no advantage over the MAC address. As it stands, IP gets a packet to the destination network by limiting routing tables to storing only network addresses allowing routing to be a simple and more efficient process.

IP addresses allow you to organize a collection of networks in a logical hierarchical fashion. There are three kinds of IP addresses:

• Unicast—Thiskind of address is the most common type of IP address. It uniquely identifies a single host on the network.

• Broadcast—Notto be confused with an Ethernet frame broadcast, IP also provides this capability by setting aside a set of addresses that can be used for broadcasting to send data to every host system on a particular network.

• Multicast—Similarto broadcast addresses, multicasting addresses send data to multiple destinations. The difference between a multicast address and a broadcast address is that a multicast address can send data to multiple networks to be received by hosts that are configured to receive the data instead of every host on the network.

• Anycast—WhenIpv6 nodes need to transmit, each individual node can transmit data to a list (or group) of addresses. This type of transmission is called Anycast and it’s fullysupported in IPv6.

Additionally, there are address classes, which are used mainly to define the size of the network and host portions of the IP address.

IP Address Classes

The Internet is a collection of networks that are all joined together by routers to create a larger network. The name itself says it all. The Internet Protocol (IP) makes this possible because it allows for addressing each network that is attached to the Internet, as well as identifying the host computers that reside on each network. When packets are routed through the Internet (or through a private corporate network that uses TCP/IP—an intranet), the IP address is used to get the data to the destination network. When the data packet is delivered to a router on the destination network, the actual hardware address (MAC address) of the computer is used to deliver the packet to the correct computer. This is done by taking the host portion of the IP address and consulting a table that maps hardware addresses to IP host addresses for the local network. If no match is found, the Address Resolution Protocol (ARP) is used on the local wire to find out the hardware address, and it is added to the table.

The important factor here is that it’s possible to assign an address both to networks and to the individual hosts.

An IP address is 4 bytes long (32 bits). Whereas MAC addresses usually are expressed in hexadecimal notation, IP addresses usually are written using dotted-decimal notation. Each byte of the entire address is converted to its decimal representation, and then the 4 bytes of the address are separated by periods to make it easier to remember. The decimal values are much easier to remember than their binary equivalents.

Because IP addresses are used to route a packet through a collection of separate networks, it is important to know what part of the IP address is used as the network address and what part is used for the host computer’s address.

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