# CST3607 Class Notes 2021-09-11

## Subnetting into a Large Number of Subnets

• Incrementing subnets using the Block Size works for a small number of subnets, but is not efficient when you need hundreds or thousands or millions of subnets. It doesn’t scale.
• Using the Base-256 conversion scales.

## Determine the network address of a high subnet number.

2. Convert the resulting number of addresses to its Base-256 (dotted-decimal) equivalent.
3. Add the Base-256 (dotted-decimal) equivalent to the network address/subnet zero, to determine the target subnet address.

• If you’re given subnet number x, then you use x as is to multiply by the number of addresses per subnet.
• If you’re given the nth, subnet, e.g. 59th, 343rd, then you subtract one, then multiply by the number of addresses per subnet. (Because we start counting from zero.)

## Subnetting Tips/Notes

• If no mask/prefix is given, then borrow bits starting from the “Class” boundary of the IP address.
• If a mask/prefix is given, then the given mask/prefix is the result of subnetting.  (Borrow bits from the “Class” boundary to the given mask/prefix.) (e.g. Q. 7, Pg. 40)
• The total number of subnets and total number of hosts must be a power of 2.
• Is the question asking for “subnets” or “hosts”
• If you’re asked for the # of hosts, then you must determine how many bits are needed to get that # of hosts, then subtract those bits from the 32 IPv4 bits, to determine the network bits / mask / prefix.
• Determine the number of subnets: 2[number of bits borrowed].
• Determine the total number of addresses: 2[the number of host bits].
• Block Size:
• The block size (256 – [The interesting octet]) is best used to determine the increment of the subnets.
• The interesting octet is the last octet, from the left, that you borrowed bits from.
• The “block size” is not the number of addresses per subnet. It is the increment from one subnet to the next, within the “interesting” octet.
• Determine how many addresses to add to the network address/subnet zero to get to the target subnet.
• 1. Multiplying (Subnet “Number”) by the (number of addresses per subnet).
(For the Nth subnet, subtract 1 before multiplying by the number of addresses per subnet.)
• 2. Convert the result to its Base-256 equivalent
• 3. Add the Base-256 equivalent to the original network address of the block to get the network/subnet address of the target subnet.
• The “subnet address” is an alternate term for the “network address” of a subnet.
• Subnet using the methods that work for all subnets, large or small. Switching methods depending on the size of the subnet requires more effort than is necessary.
• Practice makes improvement!

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## Do: Assignment #2: Due before Tues. Feb. 23, 2020 6pm EST

• Important: Make sure to read and understand the instructions on how to handle the protected PDF
• If you have any issues completing all parts of every question on the assignment, e-mail me with the question # and the specifics you need assistance with.
• No late assignments will be accepted.

## Read / Watch / Do

### CCNA Certification Study Guide, Volume 2

• Read Chapter : 6 OSPF (Open Shortest Path First)
• Do the Written Labs