Cabletron Systems EMM-E6 Ethernet manual S H

REVIEW OF ADDRESSING

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.

Network Actual (195. 191. 21. XXX) - Class C Network ID, assigned by IANA.

This class of network allows for the creation of up to 254 Host IDs on one network.

1	1	0	0	0	0	1	1	1	0	1	1	1	1	1	1	0	0	0	1	0	1	0	1
Default Subnet Mask (255. 255. 255. 0) - Masks out Network ID octets,
allows all of final octet to be used for Host IDs.
1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1

Modified Subnet Mask (255. 255. 255. 224) - Masks first three bits of fourth octet, 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.

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Subnet Logical - Shows how final five bits of original remain for Host IDs.

N N N N N N N N N N N N N N N N N N N N N N N N

Subnet Actual (195. 191. 21. 87) - Host number 23 on Subnet number 64.

1 1 0 0 0 0 1 1 1 0 1 1 1 1 1 1 0 0 0 1 0 1 0 1

X X X X

0 0 0 0

1 1 1 0

S S S H

Subnet

64

0 1 0 1

X X X X

0 0 0 0

0 0 0 0

H H H H

Host

23

0 1 1 1

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.

EMM-E6 User’s Guide 1-27