Summary of Module 9

Summary
This page summarizes the topics discussed in this module.


The U.S. Department of Defense (DoD) TCP/IP reference model has four layers: the application layer, transport layer, Internet layer, and the network access layer. The application layer handles high-level protocols, issues of representation, encoding, and dialog control. The transport layer provides transport services from the source host to the destination host. The purpose of the Internet layer is to select the best path through the network for packet transmissions. The network access layer is concerned with the physical link to the network media.

Although some layers of the TCP/IP reference model correspond to the seven layers of the OSI model, there are differences. The TCP/IP model combines the presentation and session layer into its application layer. The TCP/IP model combines the OSI data link and physical layers into its network access layer.

Routers use the IP address to move data packets between networks. IP addresses are thirty-two bits long according to the current version IPv4 and are divided into four octets of eight bits each. They operate at the network layer, Layer 3, of the OSI model, which is the Internet layer of the TCP/IP model.

The IP address of a host is a logical address and can be changed. The Media Access Control (MAC) address of the workstation is a 48-bit physical address. This address is usually burned into the network interface card (NIC) and cannot change unless the NIC is replaced. TCP/IP communications within a LAN segment require both a destination IP address and a destination MAC address for delivery. While IP address are unique and routable throughout the Internet, when a packet arrives at the destination network there needs to be a way to automatically map the IP address to a MAC address. The TCP/IP suite has a protocol, called Address Resolution Protocol (ARP), which can automatically obtain MAC addresses for local transmission. A variation on ARP called Proxy ARP will provide the MAC address of an intermediate device for transmission to another network segment.

There are five classes of IP addresses, A through E. Only the first three classes are used commercially. Depending on the class, the network and host part of the address will use a different number of bits. The Class D address is used for multicast groups. Class E addresses are reserved for research use only.

An IP address that has binary zeros in all host bit positions is used to identify the network itself. An address in which all of the host bits are set to one is the broadcast address and is used for broadcasting packets to all the devices on a network.

Public IP addresses are unique. No two machines that connect to a public network can have the same IP address because public IP addresses are global and standardized. Private networks that are not connected to the Internet may use any host addresses, as long as each host within the private network is unique. Three blocks of IP addresses are reserved for private, internal use. These three blocks consist of one Class A, a range of Class B addresses, and a range of Class C addresses. Addresses that fall within these ranges are discarded by routers and not routed on the Internet backbone.

Subnetting is another means of dividing and identifying separate networks throughout the LAN. Subnetting a network means to use the subnet mask to divide the network and break a large network up into smaller, more efficient and manageable segments, or subnets. Subnet addresses include the network portion, plus a subnet field and a host field. The subnet field and the host field are created from the original host portion for the entire network.

A more extendible and scalable version of IP, IP Version 6 (IPv6), has been defined and developed. IPv6 uses 128 bits rather than the 32 bits currently used in IPv4. IPv6 uses hexadecimal numbers to represent the 128 bits. IPv6 is being implemented in select networks and may eventually replace IPv4 as the dominant Internet protocol.

IP addresses are assigned to hosts in the following ways:

• Statically – manually, by a network administrator

• Dynamically – automatically, using reverse address resolution protocol, bootstrap protocol (BOOTP), or Dynamic Host Configuration Protocol (DHCP)