Need help with logistics

My current setup:
  We lease one half of our building (upstairs and downstairs of West half).  We are a small company of <50 employees running a W2k domain with about 70 nodes, 6 servers (1 single Exchange server), and are fairly strapped when it comes to server space ane email (running Exchange 2000 standard w/16GB database limit) space.  We have a single Full T1 coming in.  Currently only our half of the building is wired with Cat5 for phones and data.
  Now, we are talking about leasing the other half and only using a small portion for us and subleasing the remainder out to two small companies (one is only 5 people, the other will be about 15 people).  I was asked what it would entail network-wise for this to happen.  We do not want the other companies on our network (obviously).  I'm thnking that both companies will be using our current T1, router and firewall.....How can I seperate them into three seperate "entities"?
tenoverAsked:
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rhandelsCommented:
Hi,

Just one thing, VLAN's (Virtual LAN's), but this can be quite complex and you will need to have switches that can handle these kind of complex settings, you wuld need at least CISCO 2950 switches to get thos done... And even then, all traffic from them will be coming through your line, what about spam lists, do keep in mind that you will chare one public ip address with them.. Also, incoming mail can be a problem if they also have their own mailserver..

So the answer isn't quite as simple as it looks i'm afraid.. But do feel free to ask if you need some specific info.
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tenoverAuthor Commented:
No, no...I KNOW it's not easy!  I thought about VLans, but currently we're running on cheesey NetGear 10/100 non-manageable switches!  And the email thing....Yeah, unless they bring in their own T1's, then they'd have to use our mailserver.  No good.
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rhandelsCommented:
Hi,

If you only have unmanaged switches, this cannot work unfortenately.. But why are they bothering you with the networking setup of the other companies?? Best is that they have their own T1 or ADSL line (costs aren't that high) and use their own switches, else you will have to totally renew your networking config..
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rhandelsCommented:
I gave a good enough answer to get the points i guess..
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