Basic TCP/IP configuration parameters

Basic TCP/IP configuration parameters

This section describes basic TCP/IP parameters that you can configure using the Network Configuration Manager:

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Host name

The local host name for a system should be unique on your network. It can consist of lowercase letters and numbers, must begin with a letter, and should be no longer than 63 characters. mail and other programs use the host name to identify the correct data destination. Example valid machine names are scosysv, tcpdev, and account1.

Each network interface can be configured with a different host name. The host name and associated IPv4 address for the interface are added to the /etc/hosts file.

NOTE: You can also change entries in /etc/hosts using the Client Manager.

You cannot change the system name using the Network Configuration Manager. You must use the uname(1) command to do this.

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DHCP client

Configuring a system as a DHCP client allows it to obtain its other basic TCP/IP configuration parameters (domain name, IPv4 address, netmask, broadcast address, and default router) from a DHCP server at boot time.

Select Yes to enable DHCP server discovery on this interface. This interface will be configured with the configuration parameters that it receives.

NOTE: On a system with multiple LAN interfaces, only one interface can be configured to broadcast a discovery message. It is also not possible to broadcast a discovery message on an interface other than the one that is to be configured using DHCP.

Select No if you want to configure the interface using a static set of TCP/IP parameters.

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Domain name

The Internet domain name allows your network to fit into a hierarchical network structure composed of commercial organizations (.com), educational institutions (.edu), the government (.gov), the military (.mil) or miscellaneous organizations (.org). The domain name is also used when routing messages, such as mail, from machine to machine. Example domain names are (the domain name used by SCO) and (the domain name used by the University of California at Berkeley).

Base your domain name choice on the following:

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IP address

The IPv4 address identifies and differentiates your machine from all others on the network. It consists of a 32-bit binary number that is usually displayed as four octets expressed in decimal and separated by periods. An example IP address would be You must have a unique IPv4 address for each machine on your IPv4-based network. In addition, if your machine serves as a router to another network, it contains two or more network adapters and belongs to two or more networks. In this case, you must assign each adapter a unique IPv4 address on the appropriate network.

NOTE: The IPv4 address differs from a MAC (Media Access Control) address in that it is configurable. A MAC address is a 6-byte address that is unique to each physical network adapter. This non-configurable address is assigned by the adapter manufacturer.

The IPv4 address consists of two parts: a network address that identifies the network and a host address that identifies the particular host, or node.

Several classes of TCP/IP networks are available, each based on the number of hosts a network needs. Use the smallest network class that can accommodate all of your network's hosts. Many TCP/IP installations use one or more Class C networks, but some larger installations might need to use Class B.

Internet address classes

Class Maximum number of
hosts per network
Valid address ranges
A 16777214 through through
B 65534 through through
C 254 through through
Private internets only 16777214
65534 through (10/8) through (172.16/12) through (192.168/16)
Reserved - through (used if host will be assigned a valid address dynamically) through (loopback addresses) through (multicast addresses) through (future use) through (experimental) (limited broadcast)
If you are connecting your machine to a pre-existing network, the network address (for Class A, the first octet; for Class B, the first two octets; and for Class C; the first three octets) is the same as those of other machines on the network. In this case, you need only concern yourself with creating a unique host address.

If you are creating an entirely new network and you want to connect to the Internet, you must register its network address or addresses as described in ``Obtaining a domain name and an IP network number''.

If you do not want to connect to an outside network, you can choose any network address as long as it conforms to the syntax shown previously. In either case, once you determine the network address, you can then create the unique host address.

When you determine the IPv4 address, remember the following:

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The IPv4 network mask strips the network ID from the IPv4 address, leaving only the host ID. Each netmask consists of binary ones (decimal 255) to mask the network and subnet IDs and binary zeroes (decimal 0) to retain the host ID of the IPv4 address. For example, the default netmask for a Class B address is The first two octets have the value 255 as a result of setting all their bits to 1.

Always use the default netmask that the installation program prompts you for unless your network uses subnetting. If a subnet uses part of the IPv4 address, you must also mask this portion of the address. For example, for machines on a Class B network which uses the third octet for subnetting, the netmask would be

As another example, consider several subnetted class C networks which use the first three bits of the fourth octet for the subnet address and the last five bits for the host address. In this case, the netmask for all machines on the subnets would be which is obtained by setting all the network and subnet address bits to 1 (

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Broadcast address

All datagrams sent by TCP/IP may be received by all machines on a subnetwork. However, normally each host's network adapter ignores any packet that does not specify that computer as its destination. Occasionally, you might want to send a message to all machines on a particular network. To do so, select a broadcast address for your machine. A broadcast address is one in which all bits in the host portion of the IPv4 address have been set to 1 as recommended by RFC 919.

For example, the broadcast address for the class C network address n1.n2.n3.0 would be n1.n2.n3.255. The broadcast address will be different if your network is a subnetted class C network with network and subnet address n1.n2.n3.n4. For example, if the last five bits of the fourth octet are used for the host address, the broadcast address would be n1.n2.n3.n4+(16+8+4+2+1). Thus the network and subnet address with netmask would have a broadcast address of

Default router

The IPv4 address of the default router on your local subnet. The address is used to configure an entry for the router in the routing table. If your machine does not run a routing daemon, this entry allows it to send IPv4 packets via the router out of the local subnet.

© 2004 The SCO Group, Inc. All rights reserved.
UnixWare 7 Release 7.1.4 - 22 April 2004