Cabletron Systems EMM-E6 Ethernet - Manual (Page 40)

CHAPTER 1: INTRODUCTION
1-28 EMM-E6 User’s Guide
The example in Figure 1-3 masks out the three high order bits of the only
octet available for modification, the last octet. This provides for up to six
subnets and up to 30 Host IDs within each subnet. Modifying the default
mask for a Class B address (255.255.0.0) to mask out the third octet for
subnet purposes (255.255.255.0) would provide up to 254 subnets each
containing up to 254 Host IDs. Tables 1-7 and 1-6 show how using the
mask determines the subnet and host addresses that are available from an
individual octet. These tables examine the Host IDs and Subnet Addresses
available from the use of custom masks in both Class B and Class C IP
addresses. Bear in mind that Subnet Masks can only be modified for those
fields which are not assigned to a site by the IANA.
Table 1-6. Examples of Class C Subnet Masks
Decimal
Mask
Binary
Equivalent
Available Subnet
Addresses
Available
Host IDs
192 11000000 64 and 192 1 - 62
224 11100000 32, 64, 96, 128,
192, 224
1 - 30
240 11110000 16 - 240
increments of 16
1 -14
240 11110000 16 - 240
increments of 16
1 -14
248 11111000 8 - 248
increments of 8
1 - 6
252 11111100 4 - 252
increments of 4
1 and 2
254 11111110 2 - 254
increments of 2
None
255 11111111 1 - 254 None
CH1Book Page 28 Wednesday, March 20, 1996 7:48 AM