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Subnet Mask Query for a 172.16.21.10 network.

I want to create a test network with a 172.168.21.0, network, a 172.16.11.0 network  so they can communicate with each other.  Lets assume there is a router between the two.  Can someone confirm what the subnet mask should be please.
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tickleonthetum
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tickleonthetum
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1 Solution
 
Toni UranjekConsultant/TrainerCommented:
Hi tickleonthetum,

What's wrong with 255.255.255.0?

Toni
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tickleonthetumAuthor Commented:
Sorry that should be create a test network with a 172.16.21.0 and a 172.16.11.0 network....
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Toni UranjekConsultant/TrainerCommented:
Standard 255.255.255.0 should work in both cases. ;)
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lrmooreCommented:
Given your choices of networks:
>172.168.21.0
>172.16.11.0

You have many many options, but you can use basic classful mask of 16 bits 255.255.0.0, or a full 24 bit mask of 255.255.255.0 or anything in between.
If, however, you made a typo mistake in the Q, and they are really:
172.16.21.0
172.16.11.0
Then you can only use 24 or more bits in the mask: 255.255.255.0
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oBdACommented:
The subnet mask is completely up to you to choose. It just depends on how many networks or nodes per networks you need (though the 172.16 private range is usually used with a 255.255.0.0 mask, but you can't use that because your two required networks would be in the same range).
With your required networks, you can actually go up to a /20 network with a 255.255.240.0 netmask (4094 nodes, 14 subnets); this would give you to adjoining networks:
1. 172.16.0.0/20: 172.16.0.1 - 172.16.15.254
2. 172.16.16.0/20: 172.16.16.1 - 172.16.31.254
Anything more than /20 will leave at least one network free inbetween, anything less can't be done with your requirements.
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knightrider2k2Commented:
Same as above

For a 254 hosts network put 255.255.255.0 (/24) on both networks. To increase the number of hosts you can go down the line like /23, /22, /21 etc.
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lrmooreCommented:
oBdA is right, you can use /20 or better.
Too much caffeine today....
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Ron MalmsteadInformation Services ManagerCommented:
If you have a router in between you can use 24bit mask (255.255.255.0)
If you don't use a router, you can just set the mask to 16bit (255.255.0.0)...you woudn't need a router if they are on the same lan segment.

The mask determines how many network nodes or hosts can reside on the network segment...without routing.

So to be clear...
172.16.1.0 255.255.255.0 > Router < 172.16.21.0 255.255.255.0
or
172.16.1.0 255.255.0.0 > Switch < 172.16.21.0 255.255.0.0

Use a subnet calculator to find the appropriate subnet mask for the amount of hosts you require.  The router is intended to communicate between two lan segments, or to segregate your lan traffic.  Most managed switches can do the same thing using vlans, or by specifiying linked ports between two switches for allowing dhcp traffic through.  Some managed switches can also hand out dhcp scopes for ranges of ports as well.

There are alot of ways to do this.
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Ron MalmsteadInformation Services ManagerCommented:
the address mask 255.255.224.0 would give you a range of 172.16.1.0 thru 172.16.31.0.....that's 8190 hosts available.
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Fred MarshallPrincipalCommented:
"I want to create a test network with a 172.168.21.0, network, a 172.16.11.0 network  so they can communicate with each other.  Lets assume there is a router between the two.  Can someone confirm what the subnet mask should be please."

In this case, the subnet mask would have to be 255.0.0.0 because 168 and 16 are pretty far apart so must be part of your deemed, client range of addresses.
The address range would be:
172.xxx.xxx.xxx

"Sorry that should be create a test network with a 172.16.21.0 and a 172.16.11.0 network...."

In this case , the subnet mask would have to be 255.255.0.0 as 172.16 is the constant part (the "network").  Then, the address range would be:
172.16.xxx.xxx

This assumes that you want subnet masks that are all 1's and then all 0's.  In some cases that's all you can specify and in others you can get complex and mix ones and zeros in the mask.  
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Fred MarshallPrincipalCommented:
Some responded asking what's wrong with 255.255.255.0?

Here's the answer:

With 255.255.255.0 the first three octets are deemed the network address and the last octet is the set of client (computer/device) addresses.

So, with 172.168.21.0, and172.16.11.0, there would be two separate networks, 172.168.21 and 172.16.11.  And, the trailing zeroes in the fourth octet are unusable for clients as "0" represents the network (always).  Also, whatever the subnet mask then the last address in the client range is a "broadcast" address and is unusable for any client.

If one were to assign other addresses like:
172.168.21.12  and 172.16.11.13
then there would be clients with unique addresses "12" and "13" but on different networks.  Thus, a packet on 172.168.21.12 destined for "13" would have a full IP address destination of 172.168.21.13 and a packet on 172.16.11.13 destined for "12" would have a full IP adderss destination of 172.16.11.12.  Neither destination is a client we have identified here....
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Ron MalmsteadInformation Services ManagerCommented:
255.255.224.0  is the "closest match" for a proper mask to encompass the range you specified.  Although 16 bit mask would work just fine.  If there is routing between the two networks the mask you would likely be using a 24bit - 255.255.255.0


the address mask 255.255.224.0 would give you a range of 172.16.1.0 thru 172.16.31.0
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knightrider2k2Commented:
>>Some responded asking what's wrong with 255.255.255.0?
Fmarshall,
What are you talking about??

172.168.21.0 and 172.16.11.0 are two networks not two hosts.
Subnet 255.255.255.0 has nothing wrong with it, if there is a router with routing table between them.

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Fred MarshallPrincipalCommented:
knightrider2k2,

I tried to explain it clearly ....

There's nothing "standard" about 255.255.255.0, it's just very common.
Or you could use the notation /24 and realize that /23 an /25 are just as usable in the right situations.

But, I'd missed mention of there being a router between the networks.
So, taking that into consideration, the answer would be more likely 255.255.255.0 *in both cases* because there isn't one subnet mask but two - one for each subnet according to whatever one wants or needs.  There's nothing to suggest that one might not be /25 or /26 or ..... when the base is as given.  


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Computer101Commented:
Forced accept.

Computer101
Community Support Moderator
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