XAVI Technologies Corp. X7968r User Manual
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X7968r / X7967r / X7927r / X7922r User’s Guide76Class C networks are the smallest, only able to hold 254 hosts at most, but the total possiblenumber of class C networks exceeds 2 million (2,097,152 to be exact). LANs connected to theInternet are usually class C networks.Some important notes regarding IP addresses:The class can be determined easily from field1:field1 = 1-126: Class Afield1 = 128-191: Class Bfield1 = 192-223: Class C(field1 values not shown are reserved for special uses)A host ID can have any value except all fields set to 0 or all fields set to 255, asthose values are reserved for special uses.Subnet masksDefinitionmaskA mask looks like a regular IP address, but contains a pattern ofbits that tells what parts of an IP address are the network ID andwhat parts are the host ID: bits set to 1 mean "this bit is part of thenetwork ID" and bits set to 0 mean "this bit is part of the host ID."Subnet masks are used to define subnets (what you get after dividing a network into smallerpieces). A subnet's network ID is created by "borrowing" one or more bits from the host IDportion of the address. The subnet mask identifies these host ID bits.For example, consider a class C network 192.168.1. To split this into two subnets, you woulduse the subnet mask:255.255.255.128It's easier to see what's happening if we write this in binary:11111111. 11111111. 11111111.10000000As with any class C address, all of the bits in field1 through field3 are part of the network ID,but note how the mask specifies that the first bit in field4 is also included. Since this extra bithas only two values (0 and 1), this means there are two subnets. Each subnet uses theremaining 7 bits in field4 for its host IDs, which range from 1 to 126 hosts (instead of theusual 0 to 255 for a class C address).Similarly, to split a class C network into four subnets, the mask is:255.255.255.192 or 11111111. 11111111. 11111111.11000000The two extra bits in field4 can have four values (00, 01, 10, 11), so there are four subnets.Each subnet uses the remaining six bits in field4 for its host IDs, ranging from 1 to 62.NoteSometimes a subnet mask does not specify any additionalnetwork ID bits, and thus no subnets. Such a mask is called adefault subnet mask. These masks are:Class A: 255.0.0.0Class B: 255.255.0.0Class C:255.255.255.0These are called default because they are used when a network isinitially configured, at which time it has no subnets. |
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