 |
|
Subnet masks never have fewer ones than the masks listed in
Figure 2-3. A Class C address, for example, cannot have a subnet
mask of 255.255.0.0. Request for Comment (RFC) 950 first defined the subnetting of IP
addresses and does not allow the use of the all-zeros and all-ones
subnet, so we will initially look at subnetting examples that obey
these restrictions. In later examples, we will see how we can remove
these restrictions with the use of an appropriate routing protocol,
such as OSPF. The number of subnet bits cannot be one because of the
restriction in RFC 950 (see Tables 2-4, 2-5, and 2-6). A 1-bit subnet mask would have a value of
either zero (all zeros) or one (all ones) and this is not
allowed. |
| |