Initial /etc/inet/hosts File
When you run the Solaris installation program on a machine, the program configures the initial /etc/inet/hosts file. This file contains the minimum entries that the local host requires. The entries include the loopback address, the host IPv4 address, and the host name.
For example, the Solaris installation program might create the following /etc/inet/hosts file for machine tenere shown in Figure 4-1:
Example 5-1 /etc/inet/hosts File for Machine ahaggar
127.0.0.1 localhost loghost #loopback address 192.9.200.3 tenere #host name |
Loopback Address
In Example 5-1, the IPv4 address 127.0.0.1 is the loopback address. The loopback address is the reserved network interface that is used by the local machine to allow interprocess communication. This enables the host to send packets to itself. The ifconfig command uses the loopback address for configuration and testing, as explained in "ifconfig Command". Every machine on a TCP/IP network must use the IP address 127.0.0.1 for the local host.
Host Name
The IPv4 address 192.9.200.1 and the name tenere are the address and host name of the local machine. They are assigned to the machine's primary network interface.
Multiple Network Interfaces
Some machines have more than one network interface, because they are either routers or multihomed hosts. Each additional network interface that is attached to the machine requires its own IPv4 address and associated name. When you configure a router or multihomed host, you must add this information manually to the router's /etc/inet/hosts file. See "Configuring Routers" for more information on configuring routers and multihomed hosts.
Example 5-2 is the /etc/inet/hosts file for machine timbuktu that is shown in Figure 4-1.
Example 5-2 /etc/inet/hosts File for Machine timbuktu
127.0.0.1 localhost loghost 192.9.200.70 timbuktu #This is the local host name 192.9.201.10 timbuktu-201 #Interface to network 192.9.201 |
With these two interfaces, timbuktu connects networks 192.9.200 and 192.9.201 as a router.
How Name Services Affect the hosts Database
The NIS, NIS+, and DNS name services (or LDAP as a name service) maintain host names and addresses on one or more servers. These servers maintain hosts databases that contain information for every host and router (if applicable) on the servers' network. Refer to System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP) and System Administration Guide: Naming and Directory Services (FNS and NIS+) for more information about these services.
When Local Files Provide Name Service
On a network that uses local files for name service, machines that run in local files mode consult their individual /etc/inet/hosts files for IPv4 addresses and host names of other machines on the network. Therefore, these machine's /etc/inet/hosts files must contain the following:
Loopback address
IPv4 address and host name of the local machine (primary network interface)
IPv4 address and host name of additional network interfaces that are attached to this machine, if applicable
IPv4 addresses and host names of all hosts on the local network
IPv4 addresses and host names of any routers that this machine must know about, if applicable
IPv4 address of any machine your machine wants to refer to by its host name
The figure below shows the /etc/inet/hosts file for machine tenere. This machine runs in local files mode. Notice that the file contains the IPv4 addresses and host names for every machine on the 192.9.200 network. The file also contains the IPv4 address and interface name timbuktu-201. This interface connects the 192.9.200 network to the 192.9.201 network.
A machine that is configured as a network client uses the local /etc/inet/hosts file for its loopback address and IPv4 address.
Figure 5-1 /etc/inet/hosts File for Machine Running in Local Files Mode
ipnodes Database
The ipnodes database contains the IPv6 addresses and host names of machines on your network. If you use the NIS, NIS+, or DNS name services (or LDAP as a name service), the ipnodes database is maintained in a database that is designated for host information. For example, on a network that runs NIS+, the ipnodes database is maintained in the host table. For more information about the ipnodes database, see "/etc/inet/ipnodes File".
netmasks Database
You need to edit the netmasks database as part of network configuration only if you have set up subnetting on your network. The netmasks database consists of a list of networks and their associated subnet masks.
Note - When you create subnets, each new network must be a separate physical network. You cannot apply subnetting to a single physical network.
What Is Subnetting?
Subnetting is a method for maximizing the limited 32-bit IPv4 addressing space and reducing the size of the routing tables in a large internetwork. With any address class, subnetting provides a means of allocating a part of the host address space to network addresses, which lets you have more networks. The part of the host address space that is allocated to new network addresses is known as the subnet number.
In addition to making more efficient use of the IPv4 address space, subnetting has several administrative benefits. Routing can become very complicated as the number of networks grows. A small organization, for example, might give each local network a class C number. As the organization grows, the administration of a number of different network numbers could become complicated. A better idea is to allocate a few class B network numbers to each major division in an organization. For instance, you could allocate one to Engineering, one to Operations, and so on. Then, you could divide each class B network into additional networks, using the additional network numbers gained by subnetting. This division can also reduce the amount of routing information that must be communicated among routers.
Creating the Network Mask for IPv4 Addresses
As part of the subnetting process, you need to select a network-wide netmask. The netmask determines how many and which bits in the host address space represent the subnet number and how many bits and which represent the host number. Recall that the complete IPv4 address consists of 32 bits. Depending on the address class, as many as 24 bits and as few as 8 bits can be available for representing the host address space. The netmask is specified in the netmasks database.
If you plan to use subnets, you must determine your netmask before you configure TCP/IP. If you plan to install the operating system as part of network configuration, the Solaris installation program requests the netmask for your network.
As described in "Administering Network Numbers", 32-bit IP addresses consist of a network part and a host part. The 32 bits are divided into 4 bytes. Each byte is assigned to either the network number or the host number, depending on the network class.
For example, in a class B IPv4 address, the 2 bytes on the left are assigned to the network number, and the 2 bytes on the right are assigned to the host number. In the class B IPv4 address 129.144.41.10, you can assign the 2 bytes on the right to hosts.
If you are to implement subnetting, you need to use some of the bits in the bytes that are assigned to the host number to apply to subnet addresses. For example, a 16-bit host address space provides addressing for 65,534 hosts. If you apply the third byte to subnet addresses and the fourth to host addresses, you can address up to 254 networks, with up to 254 hosts on each network.
The bits in the host address bytes that are applied to subnet addresses and those applied to host addresses are determined by a subnet mask. Subnet masks are used to select bits from either byte for use as subnet addresses. Although netmask bits must be contiguous, they need not align on byte boundaries.
The netmask can be applied to an IPv4 address by using the bitwise logical AND operator. This operation selects out the network number and subnet number positions of the address.
Netmasks can be explained in terms of their binary representation. You can use a calculator for binary-to-decimal conversion. The following examples show both the decimal and binary forms of the netmask.
If a netmask 255.255.255.0 is applied to the IPv4 address 129.144.41.101, the result is the IPv4 address of 129.144.41.0.
129.144.41.101 & 255.255.255.0 = 129.144.41.0
In binary form, the operation is as follows:
10000001.10010000.00101001.01100101 (IPv4 address)
ANDed with
11111111.11111111.11111111.00000000 (netmask)
Now the system looks for a network number of 129.144.41 instead of a network number of 129.144. If your network has the number 129.144.41, that number is what the system checks for and finds. Because you can assign up to 254 values to the third byte of the IPv4 address space, subnetting lets you create address space for 254 networks, where previously space was available for only one.
If you are providing address space for only two additional networks, you can use the following subnet mask:
255.255.192.0
This netmask provides the following result:
11111111.11111111.1100000.00000000
This result still leaves 14 bits available for host addresses. Because all 0s and 1s are reserved, a minimum of 2 bits must be reserved for the host number.