With 16 bits total, 2 16 combinations are possible, yielding 65, Class B addresses. Recall that two of those numbers, the lowest and highest values, are reserved for special purposes. Therefore, each Class B address can support up to 65, hosts. Although it is significantly smaller than the networks created by Class A addresses, a logical group of more than 65, hosts is still unmanageable and impractical.
Therefore, like Class A networks, Class B addresses are subnetted to improve efficiency. Because the first 2 bits of a Class B address are always 10, 14 bits are left in the network portion of the address, resulting in 2 14 or 16, Class B networks. The first octet of a Class B address offers 64 possibilities, to The second octet has possibilities, 0 to That yields 16, addresses, or 25 percent of the total IP space.
Nevertheless, given the popularity and importance of the Internet, these addresses have run out quickly. This essentially leaves only Class C addresses available for new growth. A Class C address begins with binary Therefore, the lowest number that can be represented is , decimal If an IPv4 address contains a number in the range of to in the first octet, it is a Class C address.
Class C addresses were originally intended to support small networks. The first three octets of a Class C address represent the network number. The last octet may be used for hosts. One host octet yields 2 8 possibilities. After the all-0s network number and the all-1s broadcast address are subtracted, only hosts may be addressed on a Class C network. Whereas Class A and Class B networks prove impossibly large without subnetting, Class C networks can impose an overly restrictive limit on hosts.
Because the first 3 bits of a Class C address are always , 21 bits are left in the network portion of the address, resulting in 2 21 or 2,, Class C networks. With 2,, total network addresses containing a mere hosts each, Class C addresses account for Because Class A and B addresses are nearly exhausted, the remaining Class C addresses are all that is left to be assigned to new organizations that need IP networks.
Table summarizes the ranges and availability of the three address classes used to address Internet hosts. A Class D address begins with binary in the first octet. Therefore, the first octet range for a Class D address is to , or to Class D addresses are not used to address individual hosts.
Instead, each Class D address can be used to represent a group of hosts called a host group, or multicast group. Members of this group still have unique IP addresses from the Class A, B, or C range, but they also listen for messages addressed to The octet designates the address as a Class D address.
Therefore, a single routing update message can be sent to A single message sent to several select recipients is called a multicast. Class D addresses are also called multicast addresses.
A multicast is different from a broadcast. Every device on a logical network must process a broadcast, whereas only devices configured to listen for a Class D address receive a multicast. If the first octet of an IP address begins with , the address is a Class E address. Therefore, the first octet range for Class E addresses is to , or to Class E addresses are reserved for experimental purposes and should not be used to address hosts or multicast groups.
Subnet masking, or subnetting, is used to break one large group into several smaller subnetworks, as shown in Figure These subnets can then be distributed throughout an enterprise. This results in less IP address waste and better logical organization. Formalized with RFC in , subnetting introduced a third level of hierarchy to the IPv4 addressing structure.
The number of bits available to the network, subnet, and host portions of a given address varies depending on the size of the subnet mask. A subnet mask is a bit number that acts as a counterpart to the IP address. Each bit in the mask corresponds to its counterpart bit in the IP address. Logical ANDing is applied to the address and mask. 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. The 24 bits in the remaining three octets represent the hosts ID and allows for approximately 17 million hosts per network. Class A network number values begin at 1 and end at Class B addresses are for medium to large sized networks.
Class B allows for 16, networks by using the first two octets for the network ID. The first two bits in the first octet are always 1 0. The remaining six bits, together with the second octet, complete the network ID. The 16 bits in the third and fourth octet represent host ID and allows for approximately 65, hosts per network.
Class B network number values begin at and end at Class C addresses are used in small local area networks LANs. Class C allows for approximately 2 million networks by using the first three octets for the network ID. In a class C IP address, the first three bits of the first octet are always 1 1 0. And the remaining 21 bits of first three octets complete the network ID. The last octet 8 bits represent the host ID and allows for hosts per network.
Class C network number values begins at and end at A class B network number uses 16 bits for the network number and 16 bits for host numbers. The first byte of a class B network number is in the range In the number The last two bytes, Figure graphically illustrates a class B address. Class B is typically assigned to organizations with many hosts on their networks.
Class C network numbers use 24 bits for the network number and 8 bits for host numbers. Class C network numbers are appropriate for networks with few hosts--the maximum being A class C network number occupies the first three bytes of an IP address.
Only the fourth byte is assigned at the discretion of the network owners. Figure graphically represents the bytes in a class C address. The first byte of a class C network number covers the range The second and third each cover the range 1- A typical class C address might be The first three bytes,
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