Figure 1 illustrates the immediately usable address space with such a subnet mask (the dark shaded area).
* Address space where the subnet field is either 0 or -1 under one subnet mask can be used with another (wider) subnet mask.
* Under a given subnet mask, addresses where the subnet or host fields are either 0 or -1 cannot be assigned to hosts or physical networks [2, sect.
* The address space assigned under one subnet mask cannot be allocated under another (wider) subnet mask.
This is under a 26-bit-wide subnet mask (OxffffffcO).
3 The reason this becomes 7 usable subnets and not 6 is that the subnet x.x.x.56 - x.x.x.63 does not have -1 in the subnet field under a 29-bit-wide subnet mask (Oxfffffff8)--rather the subnet field becomes 0x38 (decimal 56), which has two leading zero bits.
Subnetting is done by "extending" the network part of the address to cover part of the host portion of the address, and the bitmask covering the old network part and the "extended" network part is called the subnet mask. The part of the host portion of the IP address covered by the subnet mask.
This is also an area in which a mask-based routing protocol can help increase the address space utilization, since it opens up for use of so-called variable-length subnet masks (VLSMs).