CHAPTER 1: INTRODUCTION
Page 1-18
EMME USER’S GUIDE
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.
Table 1-5. Class and Default Masks
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.
Table 1-6. Examples of Subnet Masks
Network Class
Length of Network ID
Default Mask
Class A
X.
255.0.0.0
Class B
X.X.
255.255.0.0
Class C
X.X.X.
255.255.255.0
Decimal
Mask
Binary
Equivalent
Available Subnet
Addresses
Available
Host IDs
255
11111111
1 - 254
None
254
11111110
2 - 254 (Even numbers only)
None
252
11111100
4 - 252 (Multiples of 4 only)
1 and 2
248
11111000
8 - 248 (Multiples of 8 only)
1 - 6
240
11110000
16 - 240 (Multiples of 16 only)
1 -14
224
11100000
32, 64, 96, 128, 192, 224
1 - 30
192
11000000
64 and 192
1 - 62
