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Subnets and reserved addresses for a Class C network

Posted on 2003-11-05
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Just a basic IPv4 question about subnets. Simply dealing with a Class C network, let's assume that we are on network segment 192.168.1.xxx. I understand that the IP address of 192.168.1.0 is reserved for representing the entire netowrk segment for devices such as routers. If we wanted to break our Class C LAN into 4 subnets using a subnet mask of 255.255.255.192, our subnets would then be broken down as follows:

Subnet 1: 192.168.1.0 - 192.168.1.63
Subnet 2: 192.168.1.64 - 192.168.1.127
Subnet 3: 192.168.1.128 - 192.168.1.191
Subnet 4: 192.168.1.192 - 192.168.1.255

I also understand that 192.168.1.255 is reserved for transmitting a signal to the entire segment. This is where my question lies: Is 192.168.1.255 used to transmit a signal to all computers on 192.168.1.xxx or does 192.168.1.63, 192.168.1.127, 192.168.1.191 become reserved for each subnet and are used to transmit to all computers on the corresponding subnet to which the belong? If that is true would we need a router attached to each subnet, or can each subnet access a single router on subnet 1, for example, on 192.168.1.1?

If I am way off with my defining of subnets as well please set me straight.

Thanks for your help!

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Question by:ucalgaryj
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JFrederick29 earned 50 total points
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Each subnet would have its own broadcast address: (Last Address in Subnet)

Broadcast Addresses:

Subnet 1: 192.168.1.63
Subnet 2: 192.168.1.127
Subnet 3: 192.168.1.191

Yes, you need a router to communicate between subnets.  You can use a single physical interface and create logical subinterfaces to achieve this without extra hardware.

So your subnets would be the following with a possible router/gateway address associated:

192.168.1.0
255.255.255.192
Gateway=192.168.1.1

192.168.1.64
255.255.255.192
Gateway=192.168.1.65

192.168.1.128
255.255.255.192
Gateway=192.168.1.129
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by:JFrederick29
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You could also use the following subnet as long as your router supports it:

192.168.1.192
255.255.255.192
Gateway=192.168.1.193
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by:ihuckaby
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In any subnet, the lowest address is your network address, and the highest address is your broadcast address.

After subnetting, a machine on subnet 1 would broadcast to 192.168.1.63, subnet 2 would broadcast to 192.168.1.127, etc.

After subnetting as you laid it out, 192.168.1.255 would only go out to subnet 4.

If you want to connect the subnets, you would need a router that is configured to be in each subnet (not a router for each subnet, but one that is connected to all of them.)

For example, if you have one router that has 192.168.1.1, 192.168.1.65, 192.168.1.129, and 192.168.1.193 all as interfaces, with the appropriate subnet connected to each interface:  THEN said router could transmit packets between the subnets.  That's what routers do, they connect different networks (and subnets are essentially different networks).

On a side note, just because you are asking about broadcasts. . .  Broadcasts are not routed under normal circumstances.  One of the primary purposes of subnetting is to limit the size of broadcast domains (a collection of all the devices that will hear a given broadcast).  Therefore, your broadcast address is not going to be able to connect to all four subnets, just the subnet on which it resides.

If you want all your machines to communicate through broadcast, then the easiest answer is not to subnet in the first place.

I'm not sure exactly what you're trying to accomplish, so I'm not sure if I've answered your underlying question.  Be happy to follow up if there's something I've left out.
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by:ucalgaryj
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Thanks for the reponse ihuckaby. JFrederick29 answered it first so I gave him credit, but thank you for the detail on broadcasts and how to configure the router. I actually teach a basic networking course at a univeristy and the question came up during lecture and I was unsure of the answer. Thanks again!
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