The fourth column shows how many valid hosts are possible on each of these subnets. The third column shows how many subnets are possible. The second column shows the resulting subnet mask in dotted decimal format. The first column shows how many bits are borrowed from the host portion of the address for subnetting. There are three tables, one for each class of addresses. 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. A subnet mask defines which portion of the address is used to identify the network and which denotes the hosts. This is done by borrowing bits from the host portion of the IP address, enabling more efficient use of the network address. Subnetting is the concept of dividing the network into smaller portions called subnets. Yet, when advertising to other providers, the ISP only needs to advertise 172.16.0.0/16. This allows for the summarization of the domains to be done at the higher level. CIDR also depicts a more hierarchical Internet architecture, where each domain takes its IP addresses from a higher level. Consider routing table of an organizations router shown below: Subnet Number Subnet Mask Next Hop. Length means the number of left-most contiguous mask bits that are set to one. In CIDR, an IP network is represented by a prefix, which is an IP address and some indication of the length of the mask. It was needed because of the rapid growth of the Internet and growth of the IP routing tables held in the Internet routers.ĬIDR moves way from the traditional IP classes (Class A, Class B, Class C, and so on). Multicast IP addresses have their first octets in the range 224 to 239.Ĭlass E - Reserved for experimental usage and includes the range of addresses with a first octet from 240 to 255.Ĭlassless Interdomain Routing (CIDR) was introduced to improve both address space utilization and routing scalability in the Internet. The first octet range of 192 to 223 is a Class C address.Ĭlass D - Used for multicast. Any address whose first octet is in the range 128 to 191 is a Class B address.Ĭlass C - The first three octets denote the network address, and the last octet is the host portion. Note that 0 is reserved as a part of the default address, and 127 is reserved for internal loopback testing.Ĭlass B - The first two octets denote the network address, and the last two octets are the host portion. Any IP address whose first octet is between 1 and 126 is a Class A address. The following are the classes of IP addresses.Ĭlass A - The first octet denotes the network address, and the last three octets are the host portion. Problem: What are the Classes of Subnets & Hosts for structuring IP addresses?
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