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http:\\ciscocertifications.info
CLASSFIRSTOCTETNETWORKIDDEFAULTSUBNET MASKAVAILABILITY
A
1-126
First Octet
255.0.0.0
AVAILABLE
B
128-191
First 2 Octets
255.255.0.0
AVAILABLE
C
192-223
First 3 Octets
255.255.255.0
AVAILABLE
D
224-239
N/A
N/A
RESERVED FOR
MULTICASTING
E
240-255
N/A
N/A
RESERVED
Note: 127 is reserved for loopback (127.0.0.1) and is used for internal testing on the local machine.
Using this table we can see the IP address in our above example is a Class C address. We can also see
which part of that IP address is the Network ID and which is the Host ID.
Network ID: (First 3 Octets) = 193.10.30
Host ID: (However many Octets are left) = 2
Whenever you want to refer to your entire network with an IP address, the host section is set to all 0's
(binary=00000000) = 0. For example 193.10.30.0 specifies the network for the above address. When
the host section is set to all 1's (binary=11111111) = 255, it specifies a broadcast that is sent to all
hosts on a network. 193.10.30.255 specifies a broadcast address for our example IP address.
*Keypoints:Know the range of IP address classes and their default subnet mask.Class A IP addresses allow the most number of hosts.Class C IP addresses allow the fewest number of hosts.Know the range for Class D addresses and that these are for a multicast group.Subnetting
Subnetting is the process used to divide the total available IP addressed (hosts) for your Network into
smaller subnetworks (subnets). For example, the Network ID we used in the discussion above
(193.10.30.0). This network would consist of 256 possible IP addresses (193.10.30.0 -
193.10.30.255). We know this because in a Class C address, only the last octet is available for host
IDs (0000000 - 11111111) or (0-255). Since 0 is used to identify the whole network and 255 is re-
served for broadcasts, that leaves us with 254 possible hosts (193.10.30.1 - 193.10.30.254).
Suppose we wanted to divide those 254 addresses up into 6 smaller subnets. This can be done by using
what is referred to as a Subnet Mask. By looking at the above table we can see Class C addresses all
have a default subnet mask of 255.255.255.0. Since the last octet of the subnet mask is 0, it means that
the host IDs have not been subdivide into smaller subnets. However, if we choose to divide our net-
work into a few smaller segments (subnets), then we would change the default subnet mask by replac-
ing the last octet with one of the valid subnet masks.
On the exam you will be asked to calculate subnet masks, valid ranges within a subnet, number of sub-
nets possible and number of hosts possible. If you memorize the 2 tables below, you should have no
problem answering any of these questions.
Class B Addresses# of bitsSubnet maskSubnetsHostsRange
2
255.255.192.0
2
16,382
64
3
255.255.224.0
6
8190
32
4
255.255.240.0
14
4094
16
5
255.255.248.0
30
2046
8