Understanding Default Subnet Masks

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Understanding Default Subnet Masks

Microsoft TCP/IP assigns a subnet mask to an IP address by default that can then be changed if needed. Table 4.5 shows the subnet mask that appears in the subnet mask field when an IP address is entered in the Microsoft TCP/IP Configuration dialog box.

In Chapter 3, the discussion of the TCP/IP addressing scheme focused on the different classes of IP addresses and the number of different networks and hosts per network that are available for each of the IP address classes. These values were based on the default subnet masks. See Table 4.6 for a summary.

If the hosts on your internetwork are not directly on the Internet, you are free to choose the network IDs that you use. For the hosts and subnets that are a part of the Internet, however, the network IDs you use must be assigned by InterNIC.

InterNIC—the Internet Network Information Center—is responsible for assigning network IDs for use on the Internet, among other things. You can visit InterNIC at www.internic.net.

If you are using network IDs assigned by InterNIC, you do not have the choice of choosing the address class you use. Using the address assigned by InterNIC, the number of subnets you use is normally limited by the number of network IDs assigned by InterNIC, and the number of hosts per subnet is determined by the class of address. Fortunately, if you are not assigned enough network addresses you can subdivide your network into a greater number of subnets by choosing the proper subnet mask. However, if you subnet your network, you have fewer possible hosts on each subnet.

Many companies today are avoiding the addressing constraints and security risks of having their hosts directly on the Internet by setting up private networks with gateway access to the Internet. Having a private network means that only the Internet gateway host needs to have an Internet address. For security, a firewall can be set up to prevent Internet hosts from directly accessing the company’s network.

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