Variable Length Subnet Masking Tutorial

Ipv6 is a 128 bit addressing scheme that will allow for significant expansion to occur as the internet continues to grow, but IPv4 is still has a big part to play. Firstly, IPv4 addressing is a lot less complicated than IPv6 addressing, plus we are now utilising IPv4 addresses more efficiently thanks to variable length subnet masks (VLSM) which help reduce the amount of unusable addresses that was commonplace with the old fashioned method of subnetting widely used back in the 1990's.

VLSM allows you to use more than one subnet mask within a network. By doing this, it takes the wasted addresses within a subnet and places them in a smaller subnet so they can be used. A common scenario would see large subnets being used for the hosts on a local area network (LAN) and a number of small subnets used for addressing on the wide area network (WAN) links that connect the LANs together.

Here is an example of a subnet within a subnet:

Subnet 1: 192.168.1.0/27 (Reserved for Network Address) 192.168.1.1/27 to 192.168.1.30/27 (Range of usable addresses) 192.168.1.31/27 (Reserved for Broadcast Address)

Subnet 2: 192.168.1.32/27 (Reserved for Network Address) 192.168.1.33/27 to 192.168.1.62/27 (Range of usable addresses) 192.168.1.63/27 (Reserved for Broadcast Address)

Subnet 3: 192.168.1.64/27 (Subnetted into 8 smaller subnets)

Small subnet 1: 192.168.1.64/30 (Reserved for Network Address) 192.168.1.65/30 to 192.168.1.66/30 (Range of usable addresses) 192.168.1.67/30 (Reserved for Broadcast Address)

Small subnet 2: 192.168.1.68/30 (Reserved for Network Address) 192.168.1.69/30 to 192.168.1.70/30 (Range of usable addresses) 192.168.1.71/30 (Reserved for Broadcast Address)

Small subnet 3: 192.168.1.72/30 (Reserved for Network Address) 192.168.1.73/30 to 192.168.1.74/30 (Range of usable addresses) 192.168.1.75/30 (Reserved for Broadcast Address)

Small subnet 4: 192.168.1.76/30 (Reserved for Network Address) 192.168.1.77/30 to 192.168.1.78/30 (Range of usable addresses) 192.168.1.79/30 (Reserved for Broadcast Address)

Small subnet 5: 192.168.1.80/30 (Reserved for Network Address) 192.168.1.81/30 to 192.168.1.82/30 (Range of usable addresses) 192.168.1.83/30 (Reserved for Broadcast Address)

Small subnet 6: 192.168.1.84/30 (Reserved for Network Address) 192.168.1.85/30 to 192.168.1.86/30 (Range of usable addresses) 192.168.1.87/30 (Reserved for Broadcast Address)

Small subnet 7: 192.168.1.88/30 (Reserved for Network Address) 192.168.1.89/30 to 192.168.1.90/30 (Range of usable addresses) 192.168.1.91/30 (Reserved for Broadcast Address)

Small subnet 8: 192.168.1.92/30 (Reserved for Network Address) 192.168.1.93/30 to 192.168.1.94/30 (Range of usable addresses) 192.168.1.95/30 (Reserved for Broadcast Address)

Subnet 4: 192.168.1.96/27 (Reserved for Network Address) 192.168.1.97/27 to 192.168.1.126/27 (Range of usable addresses) 192.168.1.127/27 (Reserved for Broadcast Address)

Subnet 5: 192.168.1.128/27 (Reserved for Network Address) 192.168.1.129/27 to 192.168.1.158/27 (Range of usable addresses) 192.168.1.159/27 (Reserved for Broadcast Address)

Subnet 6: 192.168.1.160/27 (Reserved for Network Address) 192.168.1.161/27 to 192.168.1.190/27 (Range of usable addresses) 192.168.1.191/27 (Reserved for Broadcast Address)

Subnet 7: 192.168.1.192/27 (Reserved for Network Address) 192.168.1.193/27 to 192.168.1.222/27 (Range of usable addresses) 192.168.1.223/27 (Reserved for Broadcast Address)

Subnet 8: 192.168.1.224/27 (Reserved for Network Address) 192.168.1.225/27 to 192.168.1.254/27 (Range of usable addresses) 192.168.1.255/27 (Reserved for Broadcast Address)

As we can see, all subnets other than subnet 3 have 30 usable IP addresses; subnet 3 has been divided into 8 smaller subnets, each containing 2 usable IP addresses, which is the perfect number for WAN links.

You may be wondering, what's with the /27 and /30 suffixes? Where's the subnet mask? The /27 and /30 suffixes are just another way of expressing the subnet mask, using VLSM. Here's how it works:

11111111.11111111.11111111.11111111 00000000.00000000.00000000.00000000

There are 32 bits in a subnet mask with 4 octets containing 8 bits each.

Here's a table that shows you the decimal value of each bit in an octet:

128

64

32

16

8

4

2

1

We can tell by using this table as a reference that:

The 1st bit in an octet is worth 128 The 2nd bit in an octet is worth 64 The 3rd bit in an octet is worth 32 The 4th bit in an octet is worth 16 The 5th bit in an octet is worth 8 The 6th bit in an octet is worth 4 The 7th bit in an octet is worth 2 The 8th bit in an octet is worth 1

The following subnet mask: 192.168.1.0/27 looks like this in binary 11111111.11111111.11111111.11100000

We can see that 3 bits have been used to create the 8 large subnets and the remaining 5 bits are used so that each subnet can have 30 usable host addresses. In the example above, subnet 3 has been divided into 8 small subnets which use the /30 suffix and each contains 4 addresses (2 usable addresses plus the network address and the broadcast address), which is the perfect number for WAN links.

The following subnet mask: 192.168.1.64/30 looks like this in binary 11111111.11111111.11111111.11111100

Notice that 6 bits have been used for the subnets, which creates 64 subnets, but we only use 8 of them because the larger subnet (subnet 3) can only hold 32 addresses. The remaining 2 bits are used for host addresses, which gives 2 usable addresses per small subnet.

The addressing scheme used in this article will allow you to create a number of medium sized subnetworks and lots of small subnetworks; this is perfect for a business with many different deparments across multiple sites. It's secure, but more to the point, it's a much less wasteful way of creating subnets than simply using a traditional subnet mask.

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Related Articles - VLSM, IP Addressing, Subnet Mask, Computer Networking, Subnetting, Article, Tutorial,

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