Subnetting and Tables. Subnetting is the concept of dividing the network into smaller portions called subnets. This is done by borrowing bits from the host portion of the IP address, enabling more efficient use of the network address. A subnet mask defines which portion of the address is used to identify the network and which denotes the hosts.
The following tables show all possible ways a major network can be subnetted, and, in each case, how many effective subnets and hosts are possible.
There are three tables, one for each class of addresses. The first column shows how many bits are borrowed from the host portion of the address for subnetting. The second column shows the resulting subnet mask in dotted decimal format. It consists of four octets each of which can contain one to three digits ranging from 0 to separated by a single dot.
Here, each number is the decimal representation base for an 8 digit binary number base Some bits of the IP address represents the network and the remaining bits represent the host. The IP address can be further be divided into two parts:.
Network ID: It identifies which network you are on. The number of networks in any class is given by the formula:. Host ID: It identifies your machine on the network. The number of hosts in any class is given by the formula:. The IP address belonging to Class A uses only the first octet to identify the network and the last three octets are used to identify the host. The default subnet mask for Class A IP address is Subnet masks are used to tell hosts on the network which part is the network address and which part is the host address of an IP address.
The IP address bits that have corresponding mask bits as 1 represents the network ID and the address bits that have corresponding mask bits as set to 0 represent the host ID. The IP address belonging to Class B uses the first two octets to identify the network and the last two octets are used to identify the host. The first two bit of the first octet is always set to The IP address belonging to Class C uses the first three octets to identify the network and the last octet is used to identify the host.
If a bit in the IP address corresponds to a 1 bit in the subnet mask, the IP address bit represents a network number. If a bit in the IP address corresponds to a 0 bit in the subnet mask, the IP address bit represents a host number. When the subnet mask is known, it overrides the address class to determine whether a bit is either a network or a host.
This allows routers to recognize addresses differently than the format dictated by class. The mask can be used to tell hosts that although their addresses are Class B, the first three octets, instead of the first two, are the network number. In this case, the additional octet acts like part of the network number, but only inside the organization where the mask is configured.
The subnet mask applied to an address ultimately determines the network and host portions of an IP address. The network and host portions change when the subnet mask changes. If a bit mask, Therefore, the network number for this host address is The colored portion of the address shown in Figure indicates the network number.
Because the rules of class dictate that the first two octets of a Class B address are the network number, this bit mask does not create subnets within the To create subnets with this Class B address, a mask must be used that identifies bits in the third or fourth octet as part of the network number. If a bit mask such as The network number for the host in this example is The gray portion of the address shown in Figure indicates this. Routers and hosts configured with this mask see all 8 bits in the third octet as part of the network number.
These 8 bits are considered to be the subnet field because they represent network bits beyond the two octets prescribed by classful addressing. Inside this network, devices configured with a bit mask use the 8 bits of the third octet to determine to what subnet a host belongs. Because 8 bits remain in the host field, hosts may populate each network. Just as hosts must have identical network addresses, they also must match subnet fields to communicate with each other directly.
Otherwise, the services of a router must be used so that a host on one network or subnet can talk to a host on another. Figure Class B Address with Subnetting. A Class B network with an 8-bit subnet field creates 2 8 , or , potential subnets, each one equivalent to one Class C network. Two host addresses are reserved as the network number and broadcast address, respectively.
By dividing a Class B network into smaller logical groups, the internetwork can be made more manageable, more efficient, and more scalable. Notice that subnet masks are not sent as part of an IP packet header. This means that routers outside this network will not know what subnet mask is configured inside the network.
An outside router, therefore, treats In effect, subnetting classful IP addresses provides a logical structure that is hidden from the outside world. After completing this activity, you will have a better understanding of the concept of subnetting. I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands.
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