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Is the Host on My Network?

As a network associate, one of the determinations you often have to make about a host is “Is it on my network?” or “To what network does this host belong?” To make these deter-minations, you will need to determine what hosts are in a given network.

ANDing: What Is in Your Network?

Inside data network devices, digital logic is applied for their interpretation of the addresses. When creating or forwarding an IPv4 packet, the destination network address must be extracted from the destination address. This is done by a logic called AND.

The IPv4 host address is logically ANDed with its subnet mask to determine the network address to which the host is associated. When this ANDing between the address and the subnet mask is performed, the result yields the network address.

AND Operation

ANDing is one of three basic binary operations used in digital logic. The other two are OR and NOT. While all three are used in data networks, AND is used in determining the net-work address. Therefore, the discussion in this section is limited to logical AND. Logical AND is the comparison of two bits that yields the following results:

1 AND 1 = 1

1 AND 0 = 0

0 AND 1 = 0

0 AND 0 = 0

The result from anything ANDed with a 1 yields a result that is the original bit. That is, 0 AND 1 is 0, and 1 AND 1 is 1. Consequently, anything ANDed with a 0 yields a 0. These properties of ANDing are used with the subnet mask to “mask” the host bits of an IPv4 address. Each bit of the address is ANDed with the corresponding bit of the subnet mask.

Because all the bits of the subnet mask that represent host bits are 0s, the host portion of the resulting network address becomes all 0s. Recall that an IPv4 address with all 0s in the host portion represents the network address. Likewise, all the bits of the subnet mask that indi-cate the network portion are 1s. When each of these 1s is ANDed with the corresponding bit of the address, the resulting bits are identical to the original address bits.