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Appendix B Pop-up Windows, JavaScripts and Java Permissions
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Click
Content
.to show the screen below. Select the check boxes as shown in the
following screen.
Figure 153
Mozilla Firefox Content Security
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A
PPENDIX
C
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.
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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 154
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).
Table 79
IP Address Network Number and Host ID Example
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
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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.
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:
Table 80
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
255.255.255.24
8
Table 81
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
255.255.255.2
48
3 bits
2
3
– 2
6

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