“Slash notation” is the standard way to express an IP Address and Subnet Mask together outside traditional A, B, and C classes. Network professionals will refer to a Subnet Mask of “255.255.255.0” as “slash 24”, but how does one number translate to the other? Instead of writing out a full IP Address and Subnet Mask such as “192.168.100.2 255.255.255.0” it will be written as “192.168.100.2/24” which is much shorter and easier to communicate.
The number after the “/” is the number of binary ones in the Subnet Mask. A “/24” subnet mask is simply 24 binary ones in a row, adding up to “255.255.255.0”.
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Example of Slash Notation
H = Host Bit, N = Network Bit
# of new subnets = 2^N bits borrowed
# of usable addresses = (2^H bits)-2 subtract two for unusable Network ID and Broadcast ID
11111111 . 11111111 . 11111111 . 00000000 = /24 = 255.255.255.0
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NNNNNNNN . NNNNNNNN . NNNNNNNN . HHHHHHH (no H bits borrowed)
2^N bits borrowed = 1 or 2^0 = 1 subnet
(2^H bits)-2 = 254 or (2^8)-2 = 254 hosts / subnet
11111111 . 11111111 . 11111111 . 10000000 = /25 = 255.255.255.128
NNNNNNNN . NNNNNNNN . NNNNNNNN . NHHHHHH (one H bit borrowed)
2^N bits borrowed = 2 or 2^1 = 2 subnets
(2^H bits)-2 = 126 or (2^7)-2 = 126 hosts / subnet
11111111 . 11111111 . 11111111 . 11000000 = /26 = 255.255.255.192
NNNNNNNN . NNNNNNNN . NNNNNNNN . NNHHHHH (two H bits borrowed)
2^N bits borrowed = 4 or 2^2 = 4 subnets
(2^H bits)-2 = 62 or (2^6)-2 = 62 hosts / subnet
...continuing…
11111111 . 11111111 . 11111111 . 11111100 = /30 = 255.255.255.252
NNNNNNNN . NNNNNNNN . NNNNNNNN . NNNNNNHH (six H bits borrowed)
2^N bits borrowed = 64 or 2^6 = 64 subnets
(2^H bits)-2 = 2 or (2^2)-2 = 2 hosts / subnet
A “/30” subnet is the last usable subnet mask, allowing for only two usable hosts. This is commonly used in point-to-point links between sites and routers to preserve IP addresses. “/31” and “/32” subnet masks are unusable because they allow for less than two usable host addresses.
Basic Subnetting Exercise
For example, given the “192.168.100.0/24” subnet but needing two smaller subnets instead, one could cut the “/24” subnet into two separate “/25” subnets by “borrowing” one bit from the available host bits for networks.
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Subnet A (126 Usable IPs, 2 unusable IPs)
192.168.100.0/25 Network ID
192.168.100.1/25 First Usable Host IP
…Usable Host IPs…
192.168.100.126/25 Last Usable Host IP
192.168.100.127/25 Broadcast ID
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Subnet B (126 Usable IPs, 2 unusable IPs)
192.168.100.128/25 Network ID
192.168.100.129/25 First Usable Host IP
…Usable Host IPs…
192.168.100.254/25 Last Usable Host IP
192.168.100.255/25 Broadcast ID
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Instead of having one large “/24” subnet and procuring another to create the second subnet, the “/24” with 1 network, 254 usable and 2 unusable IP addresses was cut up into a “/25” with 2 networks each having 126 usable and 2 unusable IP addresses.
Unequal CIDR Subnetting Exercise
CIDR is a balancing act between the number of subnets needed and the number of usable IP addresses in those subnets. Let’s do an exercise with unequal CIDR subnets. Given the same “192.168.100.0/24” subnet create two subnets. One subnet must support 60 usable hosts, the other must support 7 usable hosts, saving as many IP addresses for future use as possible.
Standard practice is to configure the larger block of addresses first – the 60 host block. Borrowing 1 host bit will create a subnet having 126 usable addresses (too much waste). Borrowing 2 host bits will create a subnet having 62 usable address – just right. Borrowing 3 host bits will create a subnet having 30 usable addresses – not enough, so this first block will be a “/26” subnet.
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Subnet A (62 Usable IPs, 2 unusable IPs)
192.168.100.0/26 Network ID
192.168.100.1/26 First Usable Host IP
…Usable Host IPs…
192.168.100.62/26 Last Usable Host IP
192.168.100.63/26 Broadcast ID
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Untouched IPs still available
192.168.100.64 to 192.168.100.255
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Now subnet the second block – only 7 hosts. Borrowing 3 host bits will create a subnet having 30 usable addresses – too much. Borrowing 4 host bits will create a subnet having 14 usable address – a little more than needed. Borrowing 5 host bits will create a subnet having 6 usable address – not enough, so we’ll have to borrow 4 bits and have a little left over (no big deal). Here’s how the subnet table looks now…
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Subnet A (62 Usable IPs, 2 unusable IPs)
192.168.100.0/26 Network ID
192.168.100.1/26 First Usable Host IP
…Usable Host IPs…
192.168.100.62/26 Last Usable Host IP
192.168.100.63/26 Broadcast ID
-------------------------------------------------------------
Subnet B (14 Usable IPs, 2 unusable IPs)
192.168.100.64/28 Network ID
192.168.100.65/28 First Usable Host IP
…Usable Host IPs…
192.168.100.78/28 Last Usable Host IP
192.168.100.79/28 Broadcast ID
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Untouched addresses still available for further subnetting
192.168.100.80 to 192.168.100.255
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