EMM-E6 Ethernet User's Guide

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