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Appendix A Setting up Your Computer’s IP Address
eircom F1000 Modem User’s Guide
306
2
If you know your DNS server IP address(es), enter the DNS server information in the
resolv.conf
file in the
/etc
directory.
The following figure shows an example where two DNS server IP
addresses are specified.
Figure 202
Red Hat 9.0: DNS Settings in resolv.conf
3
After you edit and save the configuration files, you must restart the network card. Enter
./network
restart
in the
/etc/rc.d/init.d
directory.
The following figure shows an example.
Figure 203
Red Hat 9.0: Restart Ethernet Card
Verifying Settings
Enter
ifconfig
in a terminal screen to check your TCP/IP properties.
Figure 204
Red Hat 9.0: Checking TCP/IP Properties
nameserver 172.23.5.1
nameserver 172.23.5.2
[root@localhost init.d]# network restart
Shutting down interface eth0:
[OK]
Shutting down loopback interface:
[OK]
Setting network parameters:
[OK]
Bringing up loopback interface:
[OK]
Bringing up interface eth0:
[OK]
[root@localhost]# ifconfig
eth0
Link encap:Ethernet
HWaddr 00:50:BA:72:5B:44
inet addr:172.23.19.129
Bcast:172.23.19.255
Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST
MTU:1500
Metric:1
RX packets:717 errors:0 dropped:0 overruns:0 frame:0
TX packets:13 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:730412 (713.2 Kb)
TX bytes:1570 (1.5 Kb)
Interrupt:10 Base address:0x1000
[root@localhost]#
Page 307 / 359
eircom F1000 Modem User’s Guide
307
A
PPENDIX
B
IP Addresses and Subnetting
This appendix introduces IP addresses and subnet masks.
IP addresses identify individual devices on a network. Every networking device (including
computers, servers, routers, printers, etc.) needs an IP address to communicate across the
network. These networking devices are also known as hosts.
Subnet masks determine the maximum number of possible hosts on a network. You can also use
subnet masks to divide one network into multiple sub-networks.
Introduction to IP Addresses
One part of the IP address is the network number, and the other part is the host ID. In the same
way that houses on a street share a common street name, the hosts on a network share a common
network number. Similarly, as each house has its own house number, each host on the network has
its own unique identifying number - the host ID. Routers use the network number to send packets
to the correct network, while the host ID determines to which host on the network the packets are
delivered.
Structure
An IP address is made up of four parts, written in dotted decimal notation (for example,
192.168.1.1). Each of these four parts is known as an octet. An octet is an eight-digit binary
number (for example 11000000, which is 192 in decimal notation).
Therefore, each octet has a possible range of 00000000 to 11111111 in binary, or 0 to 255 in
decimal.
Page 308 / 359
Appendix B IP Addresses and Subnetting
eircom F1000 Modem User’s Guide
308
The following figure shows an example IP address in which the first three octets (192.168.1) are
the network number, and the fourth octet (16) is the host ID.
Figure 205
Network Number and Host ID
How much of the IP address is the network number and how much is the host ID varies according
to the subnet mask.
Subnet Masks
A subnet mask is used to determine which bits are part of the network number, and which bits are
part of the host ID (using a logical AND operation). The term “subnet” is short for “sub-network”.
A subnet mask has 32 bits. If a bit in the subnet mask is a “1” then the corresponding bit in the IP
address is part of the network number. If a bit in the subnet mask is “0” then the corresponding bit
in the IP address is part of the host ID.
The following example shows a subnet mask identifying the network number (in bold text) and host
ID of an IP address (192.168.1.2 in decimal).
By convention, subnet masks always consist of a continuous sequence of ones beginning from the
leftmost bit of the mask, followed by a continuous sequence of zeros, for a total number of 32 bits.
Table 128
Subnet Masks
1ST OCTET:
(192)
2ND
OCTET:
(168)
3RD
OCTET:
(1)
4TH OCTET
(2)
IP Address (Binary)
11000000
10101000
00000001
00000010
Subnet Mask (Binary)
11111111
11111111
11111111
00000000
Network Number
11000000
10101000
00000001
Host ID
00000010
Page 309 / 359
Appendix B IP Addresses and Subnetting
eircom F1000 Modem User’s Guide
309
Subnet masks can be referred to by the size of the network number part (the bits with a “1” value).
For example, an “8-bit mask” means that the first 8 bits of the mask are ones and the remaining 24
bits are zeroes.
Subnet masks are expressed in dotted decimal notation just like IP addresses. The following
examples show the binary and decimal notation for 8-bit, 16-bit, 24-bit and 29-bit subnet masks.
Network Size
The size of the network number determines the maximum number of possible hosts you can have
on your network. The larger the number of network number bits, the smaller the number of
remaining host ID bits.
An IP address with host IDs of all zeros is the IP address of the network (192.168.1.0 with a 24-bit
subnet mask, for example). An IP address with host IDs of all ones is the broadcast address for that
network
(192.168.1.255 with a 24-bit subnet mask, for example).
As these two IP addresses cannot be used for individual hosts, calculate the maximum number of
possible hosts in a network as follows:
Notation
Since the mask is always a continuous number of ones beginning from the left, followed by a
continuous number of zeros for the remainder of the 32 bit mask, you can simply specify the
number of ones instead of writing the value of each octet. This is usually specified by writing a “/”
followed by the number of bits in the mask after the address.
For example, 192.1.1.0 /25 is equivalent to saying 192.1.1.0 with subnet mask 255.255.255.128.
Table 129
Subnet Masks
BINARY
DECIMAL
1ST
OCTET
2ND
OCTET
3RD
OCTET
4TH OCTET
8-bit mask
11111111
00000000
00000000
00000000
255.0.0.0
16-bit mask
11111111
11111111
00000000
00000000
255.255.0.0
24-bit mask
11111111
11111111
11111111
00000000
255.255.255.0
29-bit mask
11111111
11111111
11111111
11111000
255.255.255.248
Table 130
Maximum Host Numbers
SUBNET MASK
HOST ID SIZE
MAXIMUM NUMBER OF HOSTS
8 bits
255.0.0.0
24 bits
2
24
– 2
16777214
16 bits
255.255.0.0
16 bits
2
16
– 2
65534
24 bits
255.255.255.0
8 bits
2
8
– 2
254
29 bits
255.255.255.24
8
3 bits
2
3
– 2
6
Page 310 / 359
Appendix B IP Addresses and Subnetting
eircom F1000 Modem User’s Guide
310
The following table shows some possible subnet masks using both notations.
Subnetting
You can use subnetting to divide one network into multiple sub-networks. In the following example
a network administrator creates two sub-networks to isolate a group of servers from the rest of the
company network for security reasons.
In this example, the company network address is 192.168.1.0. The first three octets of the address
(192.168.1) are the network number, and the remaining octet is the host ID, allowing a maximum
of 2
8
– 2 or 254 possible hosts.
The following figure shows the company network before subnetting.
Figure 206
Subnetting Example: Before Subnetting
You can “borrow” one of the host ID bits to divide the network 192.168.1.0 into two separate sub-
networks. The subnet mask is now 25 bits (255.255.255.128 or /25).
The “borrowed” host ID bit can have a value of either 0 or 1, allowing two subnets; 192.168.1.0 /25
and 192.168.1.128 /25.
Table 131
Alternative Subnet Mask Notation
SUBNET MASK
ALTERNATIVE
NOTATION
LAST OCTET
(BINARY)
LAST OCTET
(DECIMAL)
255.255.255.0
/24
0000 0000
0
255.255.255.128
/25
1000 0000
128
255.255.255.192
/26
1100 0000
192
255.255.255.224
/27
1110 0000
224
255.255.255.240
/28
1111 0000
240
255.255.255.248
/29
1111 1000
248
255.255.255.252
/30
1111 1100
252

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