Rev C
Page 19
11/30/2011
Copyright © 2011 by Cirus Controls, LLC. All Rights Reserved. No part of this material may be
reproduced in any form without the express written permission of Cirus Controls LLC.
A final point. There are physical subnets, and numerical subnets. A physical subnet is a
group of components that is physically tied together without traveling through a router.
(Note some routers have hubs/switches built in which confuses this subject. On these
types of routers, there are what’s called a LAN ports and a WAN port. The LAN ports tie
to the same subnet; the WAN port is the link to other subnets). One physical subnet can
have multiple numerical subnets that can run simultaneously without interfering with
each other. However, numerical subnets MUST exist on the same physical subnet.
For example: We have a hub with four computers tied to it. Their addresses and subnet
masks define two of these computers as being in one numerical subnet. The other two
are defined by their addresses and subnet masks as being in another numerical subnet.
The first two computers can communicate with each other, and the last two computers
can communicate with each other, but computers from the first group cannot
communicate with computers from the last group. Even though they are on the same
physical subnet, there is no method for them to communicate with each other.
Gateway
The gateway was discussed in the previous section. Its function is to pass messages from
its own subnet to other subnets. In order for it to receive messages from its own subnet,
its address must fall into the numerical subnet it is passing messages for. In other words,
if the gateway is responsible for passing all messages from computers on the subnet with
addresses beginning with 172.57, its address must fall in the 172.57.x.x range.
The Whole Setup
A full description of a network components TCP/IP settings are the following 5 settings:
1)
IP Address – This is the address of the computer, and must be unique on its
subnet.
2)
Subnet Mask – Used by the computer to determine if a recipients address is in its
own subnet or a different one.
3)
Default Gateway – All communication not destined for a computer inside its own
subnet is handed off to the gateway for “routing”.
4)
Primary DNS Server – A way for the computer to take a human readable address
like
www.microsoft.com
and transform it into a number like 207.46.245.230
5)
Secondary DNS Server – A backup to the primary DNS server in case it’s
unreachable.
Not all network components will have or need to have all these fields defined. For
example, Cirus Controls spreaders only have an address and a subnet mask defined. It
does not need a default gateway because it can only communicate with other machines on
its own subnet, and does not need either entry for DNS servers because it cannot take
human input to do things like browse the internet or receive mail.