Who Sends and Receives Information: The Host

If you are an experienced Solaris user, you are no doubt familiar with the term "host," a word often used as a synonym for "computer" or "machine." From a TCP/IP perspective, only two types of entities exist on a network: routers and hosts.

A router is a machine that forwards packets from one network to another. To do this, the router must have at least two network interfaces. A machine with only one network interface cannot forward packets; it is considered a host. Most of the machines you set up on a network will be hosts.

It is possible for a machine to have more than one network interface but not function as a router. This type of machine is called a multihomed host. A multihomed host is directly connected to multiple networks through its network interfaces. However, it does not route packets from one network to another.

When a host initiates communication, it is called a sending host, or the sender. For example, a host initiates communications when its user types rlogin or sends an email message to another user. The host that is the target of the communication is called the receiving host, or recipient. For example, the remote host specified as the argument to rlogin is the recipient of the request to log in.

Each host has three characteristics that help identify it to its peers on the network. These characteristics include:

• Host name

• Internet address, or IP address, the form used in this book

• Hardware address

Host Name

The host name is the name of the local machine, combined with the name of your organization. Many organizations let users choose the host names for their machines. Programs such as sendmail and rlogin use host names to specify remote machines on a network. System Administration Guide contains more information about host names.

The host name of the machine also becomes the name of the primary network interface. This concept becomes important when you set up the network databases or configure routers.

When setting up a network, you must obtain the host names of all machines to be involved. You will use this information when setting up network databases, as described in Chapter 4, Configuring TCP/IP on the Network.

IP Address

The IP address is one of the two types of addresses each machine has on a TCP/IP network that identifies the machine to its peers on the network. This address also gives peer hosts a notion of where a particular host is located on the network. If you have installed the Solaris operating environment on a machine on a network, you may recall specifying the IP address during the installation process. IP addressing is a significant aspect of TCP/IP and is explained fully in "Designing Your IP Addressing Scheme".

Hardware Address

Each host on a network has a hardware address, which also identifies it to its peers. This address is physically assigned to the machine's CPU or network interface by the manufacturer. Each hardware address is unique.

This book uses the term Ethernet address to correspond to the hardware address. Because Ethernet is the most commonly used network media on Solaris-based networks, the text assumes that the hardware address of your Solaris host is an Ethernet address. If you are using other network media, such as FDDI, refer to the documentation that came with your media for hardware addressing information.