Cabletron Systems EMM-E6 Ethernet manual Examples of Class C Subnet Masks

CHAPTER 1: INTRODUCTION

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	Binary	Available Subnet	Available
Mask	Equivalent	Addresses	Host IDs
192	11000000	64 and 192	1 - 62
224	11100000	32, 64, 96, 128,	1 - 30
		192, 224
240	11110000	16 - 240	1 -14
		increments of 16
240	11110000	16 - 240	1 -14
		increments of 16
248	11111000	8 - 248	1 - 6
		increments of 8
252	11111100	4 - 252	1 and 2
		increments of 4
254	11111110	2 - 254	None
		increments of 2
255	11111111	1 - 254	None

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