Altogether, the first three ranges in this example would be:Īll thing considered, the shortcut table provides a simple and effective way of recalling the address range starting points associated with a given custom subnet mask. This is because the “normal” end of the range (202.202.202.31) is actually the broadcast address for that subnet, where the host ID is set to all binary 1s. Add one to find the beginning of the range, and subtract one from what you might normally consider the end of the range. However, calculating the ranges means remembering that simple rule I just outlined. This is still true, and the first three subnet IDs would be: In this case, according to our table, the ranges should begin at multiples of 16. The table can still be used for these addresses, but with one small adjustment – you’ll need to add 1 to the beginning of every range and subtract one from the end to account for the network ID and broadcast address.įor example, imagine the address 202.202.202.0 with a subnet mask of 255.255.255.240. Recall that Class C addresses use only the last octet for subnetting. While the table above works perfectly for Class A and B address ranges, it does present a small issue for Class C address subnetting. So, in our example, the first three ranges of addresses would be: When a custom subnet mask of 224 is used, new ranges always start at multiples of 32 in the second octet. Now look in the Range Multiples row directly below 224, and you’ll notice the number 32. Simply look for the value associated with the custom mask in the Mask row – in this case 224. If you used the table above, you could quickly determine the multiples at which new address ranges start. So what does this table tell you? Well, imagine that you were given the network ID 10.0.0.0 with a subnet mask of 255.224.0.0. However, the pattern that exists can easily be put into a table to help you recall mask values and address range multiples quickly and easily, as illustrated in this subnetting shortcut table: Bit Of course, we could calculate these the old fashioned way, breaking everything down to binary and then working from there. For example, the first three ranges of addresses in this example would be: When a custom value of 252 is used in a mask, new address ranges always move in multiples of 4 in the third octet. For example, if you are using the network ID 131.107.0.0, and a subnet mask of 255.255.252.0, subnetting is clearly taking place in the third octet. With any given subnet mask value, there is always an associated multiple at which new subnets will start. Now that we have the long method out of the way, we can take a look at a quick shortcut. Because you cannot configure a virtual external interface with a default gateway on a different subnet, you cannot use a /32 subnet mask for a virtual external interface, such as a VLAN or Link Aggregation interface.If you’ve been watching closely, you may have noticed the very clear pattern that develops when attempting to determine address ranges. In mixed routing mode, you can only configure a 32-bit subnet mask for a physical external interface. Below excerpt describes why /32 IP address at each side of PTP links is not possible.Ī 32-bit subnet mask defines a network with only one IP address. Success rate is 100 percent (5/5), round-trip min/avg/max = 12/16/20 msĬ 192.168.0.0 is directly connected, FastEthernet0/0Īt this juncture, one might think of using 32-bit Subnet Mask (/32) for each side of PTP links. Sending 5, 100-byte ICMP Echo to 192.168.0.1, timeout is 2 seconds: We can successfully ping the far-end interface and the subnet is accurately reflected in the routing table: An ominous warning message, no doubt, but it works just fine. We can put this theory into practice by addressing a point-to-point connection between two routers as 192.168.0.0/31. Cisco IOS has supported /31 subnets for point-to-point links since release 12.2(2) T. Recall that shrinking a /30 subnet to a /31 effectively doubles the number of point-to-point links you can address from a finite range. Thirty-one-bit subnets were first proposed in RFC 3021, which was primarily motivated by the potential for public address space conservation. % Warning: use /31 mask on non point-to-point interface cautiously Router A is connected via Fast-ethernet 0/0 having ip 192.168.0.0/31 to Fast-ethernet 0/0 having ip 192.168.0.1/31 of Router B.
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