As discussed, the subnet mask provides one way to determine which bits in the IP address belong to the network ID and which bits belong to the host ID. To be sure, you must know the number of bits in the network ID. With subnetting, an IP address alone doesn't help you understand how the address can be used. This is always the case, as any good Cisco Certified Network Associate (CCNA) will tell you. I say there are six usable addresses out of eight because the lowest address is reserved as the network number and the highest address is reserved as the broadcast address for the network. In this case, you must create a subnet that uses the subnet mask to isolate your nodes as appropriate for the number of nodes you've been assigned. For example, you might be assigned a block of eight (six usable) addresses. Most of the time, you'll be assigned a small block of public IP addresses to work with. This isn't true, however, when you need public IP addresses. Inside Out Blocks of IP addresses on the public Internetįor internal networks that use private IP addresses, you'll often be able to use the standard subnet masks. Standard Subnet Masks for Classful Networks On a classful network, all the bits in the network ID portion of the IP address are set to 1 and can be presented in dotted decimal as shown in Table 24-3. If a bit is set to 0, it corresponds to a bit in the host ID that is accessible from within the subnet.īecause a subnet mask must be configured for each IP address, nodes on both classful and nonclassful networks have subnet masks. If a bit is set to 1, it corresponds to a bit in the network ID that isn't accessible from within the subnet. Nodes can see only the portions of the IP address space that aren't masked by a bit with a value of 1. The subnet mask identifies which bits of the IP address belong to the network ID and which bits belong to the host ID. As with IP addresses, the basic form is w.x.y.z. Because it is a 32-bit value, subnet masks can be expressed as an address for which each 8-bit value (octet) is written as four separate decimal values delimited by a period (dot). The mask works by blocking areas outside the subnet so that they aren't visible from within the subnet. You use a 32-bit value known as a subnet mask to configure nodes in a subnet to communicate only with other nodes on the same subnet. Thus, instead of getting a complete network address for the public Internet, your organization is more likely to get a block of consecutive IP addresses to use. This is important, especially for the crowded public IP address space where it doesn't make sense to assign the complete IP address range for a network to an individual organization. Thus, rather than having networks with hundreds, thousands, or millions of nodes, you have a subnet that is sized appropriately for the number of nodes that you use. Subnetting is designed to make more efficient use of the IP address space. This ensures the subnet is isolated and doesn't affect other subnets. Typically, such networks are also physically separated by a router. Logically, it exists separately from other networks even if hosts on those other networks share the same network ID. A subnet is a portion of a network that operates as a separate network. Anyone that works with computers should learn about subnetting and what it means.
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