Subnetting in under five minutes

Learning subnetting in under five minutes.

As part of the apprenticeships risual education offers, one of the qualifications we teach is the Networking Fundamentals to our IT apprentices. One aspect of teaching this course is helping learners to gain knowledge in TCP/IP and subnetting. Learners have mentioned that they particularly struggle with this task and have a minimal knowledge of computer networking when starting the apprenticeship. Some learners had mentioned they’d had this explained to them by their employers in the past, but they hadn’t really understood it. One way Adam found to explain this better was to firstly show a basic IP configuration of a computer and help the learners understand what each part means such as what a default gateway is, what a subnet mask is, what an IP address is and what protocol or protocols automatically configures this information. Adam then explained that subnet masks are set into different classes, such as:

  • Class A – 255.0.0.0
  • Class B – 255.255.0.0
  • Class C – 255.255.255.0
  • Class D – 255.255.255.255

 Once the learners understood this, Adam started getting the learners to break the IPv4 addresses into binary. This involved all the learners converting an example private IP address such as 192.168.1.7 in to 11000000.1010100.00000001.00000111. One done; Adam got the learners to convert the subnet mask of 255.255.255.0 into binary too which would be 11111111.11111111.11111111.00000000. The learners in the class then placed the subnet mask in binary underneath the IP Address in binary like the following:

11000000.10101000.00000001.00000111

11111111.11111111.11111111.00000000

Adam was then able to get the learners to draw a line after the last ‘1’ in the subnet mask and line it with the last binary digit of the IP address. Adam then explained that the bit to the left of the line is the network portion, and the bit to the right of the line is the host portion, like:

11000000.10101000.00000001.xxxxxxxx = Network Portion of the address.

xxxxxxxx.xxxxxxxx.xxxxxxxx.00000111 = Host portion of the address.

Adam then explained that within a simple local area network, all devices will have an IP address which is dynamically assigned by DHCP. Adam explained that the Network portion of the IP address is the same for every device, but the host portion is different for every device. Adam then explained that you could use custom subnet masks, such as 255.255.240.0. When using a default subnet mask or custom one, you can calculate the number of host addresses that you can fit within a network. This can be done by changing the zeros in the subnet masks to ones, converting it to decimal and multiplying the decimal figure in each octect. Once learners had done this a few more times with a few different examples, they were much more confident with the basics of TCP/IP.

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