B-4
occurs. If the user uses a dial-up service to access the Internet, the associated
telecommunication cost is often directly proportional to the amount of time the user
uses the network service to connect to the Internet. Therefore, the Idle Timeout is
designed so that after there is no traffic to the Internet for a pre-configured amount of
time, the connection will automatically be taken down.
IGMP
IP Group Multicast Protocol, a protocol that is used as part of the IP multicast
protocol.
Internet Access and ISP Accounts
To access the Internet, first you need to have a device (such as a router or a modem)
that you can use to connect to the Internet using a dial-up services such as modem or
ISDN or a fixed connection service such as a leased line or a frame relay network.
Then you need to find a local ISP and apply for an ISP account, so that you can get an
account name and the associated password that you need to access the Internet.
Generally speaking, only one device can directly use a modem or an ISDN line to
access the Internet. But if you use a router, you can allow multiple users to share the
router to access the Internet.
IP Address and Network Mask
IP address is a unique identifier for a device in the IP network. It consists of 2
portions: the IP network address, and the host identifier.
The IP address is a 32-bit binary pattern, which can be represented as four cascaded
decimal numbers separated by “.”: aaa.aaa.aaa.aaa, where each “aaa” can be anything
from 000 to 255, or as four cascaded binary numbers separated by “.”:
bbbbbbbb.bbbbbbbb.bbbbbbbb.bbbbbbbb, where each “b” can either be 0 or 1.
A network mask is also a 32-bit binary pattern, and consists of consecutive leading
1’s followed by consecutive
trailing 0’s, such as
11111111.11111111.11111111.00000000. Therefore sometimes a network mask can
also be described simply as “x” number of leading 1’s.
When both are represented side by side in their binary forms, all bits in the IP address
that correspond to 1’s in the network mask become part of the IP network address,
and the remaining bits correspond to the host ID. For example, if the IP address for a
device is, in its binary form,
11011001.10110000.10010000.00000111, and if its network mask is,
11111111.11111111.11110000.00000000
It means the device’s network address is
11011001.10110000.10010000.00000000, and its host ID is,
00000000.00000000.00000000.00000111.
IP Packet Filter
A feature that allows selective IP packets to be filtered (thrown away). This feature
means each and every packet to or from an outside network will be inspected to see if
it matches the user defined criteria. When there is a match, the packet will either be
filtered or forwarded depending on the configuration.