12/20/2023 0 Comments Prefix lengths and subnet mask table![]() This figure displays the specifics of the first subnet. This means that each of the 1000 subnets can support up to 16,382 hosts. ![]() Subtracting two hosts per subnet (one for the network address and one for the broadcast address) equates to 214 – 2 = 16382 hosts per subnet. Resulting /18 Subnets 18 Subnetsīorrowing 10 bits to create the subnets, leaves 14 host bits for each subnet. This figure displays the subnets resulting from borrowing 10 bits, creating subnets from 10.0.0.0/18 to 10.255.128.0/18. This figure displays the network address and the resulting subnet mask, which converts to 255.255.192.0 or 10.0.0.0/18. Number of Subnets Created Number of Subnets Created This includes 8 bits in the second octet and 2 additional bits from the third octet. As shown in the figure, you need to borrow 10 bits to create 1024 subnets (2 10 = 1024). Starting from the left to the right with the first available host bit, borrow a single bit at a time until you reach the number of bits necessary to create 1000 subnets. To create subnets, you must borrow bits from the host portion of the IPv4 address of the existing internetwork. Therefore, the small ISP will subnet the 10.0.0.0/8 network. This means there are 8 bits in the network portion and 24 host bits available to borrow toward subnetting. Each client will need plenty of space in the host portion to create its own subnets. For example, take a small ISP that requires 1000 subnets for its clients. Some organizations, such as small service providers or large enterprises, may need even more subnets. ![]() Address Range for 172.16.0.0/23 Subnet Address Range for 23 Subnet Create 1000 Subnets with a Slash 8 prefix The first address is reserved for the network address and the last address is reserved for the broadcast address, so subtracting for these two addresses (29 – 2) equals 510 available host addresses for each /23 subnet.Īs shown in the figure, the first host address for the first subnet is 172.16.0.1, and the last host address is 172.16.1.254. Resulting /23 Subnets 23 SubnetsĪfter borrowing 7 bits for the subnet, there is one host bit remaining in the third octet, and 8 host bits remaining in the fourth octet, for a total of 9 bits that were not borrowed. In decimal, the mask is represented as 255.255.254.0, or a /23 prefix, because the third octet is 11111110 in binary and the fourth octet is 00000000 in binary. In this example, when 7 bits are borrowed, the mask is extended 7 bits into the third octet. Recall that the subnet mask must change to reflect the borrowed bits. To satisfy the requirement of 100 subnets for the enterprise, 7 bits (i.e., 27 = 128 subnets) would need to be borrowed (for a total of 128 subnets), as shown in the figure. Notice there are now up to 14 host bits that can be borrowed. The figure displays the number of subnets that can be created when borrowing bits from the third octet and the fourth octet. The table highlights all the possible scenarios for subnetting a /16 prefix. The 16 bits in the host portion are available to borrow for creating subnets. This address has 16 bits in the network portion and 16 bits in the host portion. In a situation requiring a larger number of subnets, an IPv4 network is required that has more hosts bits available to borrow. This topic explains how to create subnets that each have the same number of hosts. Some subnetting is easier than other subnetting.
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