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Before sending you off to try out practice questions, I want to mention two points. First, some practice questions you encounter may not explicitly state the size of the address block you must subnet. Here’s an example: “What is the maximum number of valid subnets and usable hosts per subnet that you can get from the network 172.26.0.0 255.255.252.0?” In such cases, assume the original address block is a classful network. In this example, the IP address is 172.26.0.0, which is a class B address (because it starts with 0b10), so you can assume that the address block is /16 (172.26.0.0/16).
The second point is that some questions will ask about wildcard masks, which are similar to subnet masks, but not actually related to the topic of subnetting. We will cover wildcard masks in chapter 17 (Dynamic Routing), as well as chapters 23 and 24 (Access Control Lists). For now, you can skip any questions on a practice website that mention wildcard masks.
Classless Inter-Domain Routing (CIDR) replaced classful addressing, allowing prefix lengths outside of the traditional /8, /16, and /24.
With CIDR, an address block can be divided into smaller networks called subnets. This process is called subnetting.
Fixed-Length Subnet Masking (FLSM) subnetting divides an address block into subnets of equal size.
Variable-Length Subnet Masking (VLSM) subnetting divides an address block into subnets of varying size.
To subnet an address block, you “borrow” bits from the host portion of the address block and add them to the network portion. Whereas the network portion of the original address block cannot be changed, the borrowed bits can be changed to make different subnets.
Each additional borrowed bit doubles the number of subnets that can be made: 1 borrowed bit = 2 subnets, 2 borrowed bits = 4 subnets, 3 borrowed bits = 8 subnets, etc. However, each additional borrowed bit halves the number of addresses in each subnet because there are fewer bits in the host portion.
The five attributes of an IPv4 network are calculated in the same manner for subnets: the network address is the first address of a subnet (host portion of all 0s), the broadcast address is the last address of a subnet (host portion of all 1s), the first usable address is the first address after the network address, the last usable address is the last address before the broadcast address, and the maximum number of hosts is 2y – 2, where y is the number of host bits.
For point-to-point links (connections between two routers), either a /30 or a /31 prefix length can be used. /30 consumes four addresses (network address, broadcast address, and two host addresses), whereas /31 consumes only two addresses (two host addresses, without a network or broadcast address).
To subnet an address block using VLSM, assign the largest subnet at the start of the address block, assign the second-largest subnet after it, and repeat the process until all subnets have been assigned.
The network address of the next subnet is the address immediately after the broadcast address of the current subnet.
In a real-world situation, you should leave some room in each subnet for future growth. When doing subnetting scenarios for practice (or for the CCNA exam), be as efficient as possible (leave as few unused addresses as possible).
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