Subnet Masks - Cabletron Systems EMME User Manual

CHAPTER 1: INTRODUCTION

1.9.4 Subnet Masks

The purpose of the subnet mask is to indicate the part of the Host ID that
is being used as a Subnet address. By default no part of the Host ID is
used, and therefore, the default or "Natural Mask" masks just the octets
that comprise the Network ID. Table 1-5 shows the default masks.
Network Class
Class A
Class B
Class C
The binary 1's in the mask "mask-out" the Network ID and the 0's show
where the Host ID is. When using part of the Host ID as a subnet address,
define a subnet mask that will mask-out the bits of the Host ID that are
being used as a subnet address. The calculations for the mask must be
done at the bit level since in some cases, always in Class C addresses, the
last octet must be split into part Host ID and part Subnet address.
Table 1-6 shows how using the mask determines the subnet and Host
addresses that are available from the octet.
Decimal
Mask Equivalent
255 11111111
254 11111110
252 11111100
248 11111000
240 11110000
224 11100000
192 11000000
Page 1-18
Table 1-5. Class and Default Masks
Length of Network ID
X.
X.X.
X.X.X.
Table 1-6. Examples of Subnet Masks
Binary
1 - 254
2 - 254 (Even numbers only)
4 - 252 (Multiples of 4 only)
8 - 248 (Multiples of 8 only)
16 - 240 (Multiples of 16 only)
32, 64, 96, 128, 192, 224
64 and 192
Available Subnet
Addresses
Default Mask
255.0.0.0
255.255.0.0
255.255.255.0
Available
Host IDs
None
None
1 and 2
1 - 6
1 -14
1 - 30
1 - 62
EMME USER'S GUIDE