DHCP in Windows 2000

I have one w2k advanced server and 50 w2k professional clients.I divided my 50 clients into 3 labs.15 clients in sales lab,20 clients in marketing lab,15 clients in accounts lab.Then i created 3 scopes in DHCP.
Sales Scope: 192.168.100.10 to 192.168.100.25
Marketing Scope:192.168.100.30 to 192.168.100.50
Accounts Scope:192.168.100.60 to 192.168.100.75
When my clients communicate with dhcp they gets IP address.If sales clients communicate with dhcp they get ip from sales,marketing gets ip from marketing scope,accounts clients get ip from accounts scope.How i do it or how the DHCP knows which is sales,marketing,accounts clients.
Anurag.