3/18/2024 0 Comments Class a subnet mask tableTo calculate an inverse mask given a subnet mask, simply subtract each octet value from 255. The rationale behind an inverse mask is that logical ANDing an inverse mask and an IP address gives the host portion of the address instead of the network/subnet portion (as a standard subnet mask would), which is of more interest to certain TCP/IP functions. + The inverse mask is primarily used in Cisco access control lists (ACLs). 255.255.255.0) is like a mask on top of the IP address and specifies the hosts. For example, the class C networks were located between the addresses 192.0.0.0 and 223.255.255.255. The +1 after each number indicates the extra subnet which can be reclaimed in this way. An IP address made it possible in the past to determine which class it belonged to. In CIDR notation this is designated by a /24 following the IP address. Example: A class C network would have a subnet mask of 255.255.255.0 which means that 24 bits are used for the network. In this case you get one extra subnet per classful network, with the slight drawback that the network address of the network becomes the network address of the first subnet instead. It also introduced a new method of denoting network masks. However, on most newer routing and other networking equipment, you can specify that it use subnet zero (the command on a cisco is ip subnet-zero). This means that in classful addressing, the first and last subnets within a network are unusable, since all subnet bits would be zeroes or ones. * Generally, host bits, subnet bits, and network bits cannot consist of all ones or all zeroes without conveying special meaning to the address (network address, broadcast address, etc). Invalid mask since it leaves no host bits In class B or larger networks, CIDR, or subnet-zero enabled networks only In Class A networks, CIDR, or subnet-zero enabled networks only
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