EMM-E6 Ethernet User's Guide

REVIEW OF ADDRESSING
EMM-E6 User’s Guide 1-27
The binary 1’s in the mask “mask-out” the Network ID and the 0’s show
where the Host ID is located. 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, and in all cases for Class
C addresses, the last octet must be split into part Host ID and part Subnet
ID. Figure 1-3 below, shows the means by which a Subnet Mask blocks
bits from an IP address to determine which bits are representing a Subnet
ID and which represent a Host ID.
Figure 1-3. Subnet Masking
If you decide to modify the default Subnet Mask in order to accommodate
subnets within your network, you must determine the number of subnets
you desire and how many Host IDs will be available within each
configured Subnet.
0011 1100 1101 1111 1000 1010 XXXX XXXX
1111 1111 1111 1111 1111 1111 0111 0000
NNNN NNNN NNNN NNNN NNNN NNNN HSSS HHHH
Subnet Logical - Shows how final five bits of original remain for Host IDs.
Modified Subnet Mask (255. 255. 255. 224) - Masks first three bits of fourth octet,
Network Actual (195. 191. 21. XXX) - Class C Network ID, assigned by IANA.
0011 1100 1101 1111 1000 1010 1010 1110
Subnet Actual (195. 191. 21. 87) - Host number 23 on Subnet number 64.
Subnet
64
Host
23
1111 1111 1111 1111 1111 1111 0000 0000
Default Subnet Mask (255. 255. 255. 0) - Masks out Network ID octets,
allows all of final octet to be used for Host IDs.
allowing a portion of the Host ID to be used for Subnet identification. This particular
custom Subnet Mask allows the creation of six subnets, each having no more than
thirty hosts (see Table 1-6, below) for a maximum of 180 Host IDs.
This class of network allows for the creation of up to 254 Host IDs on one network.
CH1Book Page 27 Wednesday, March 20, 1996 7:48 AM