version of NS you are using. However, basically, you would create a new site, and manually populate the subnet configuration to only use select subnets. Then manually assign a package\site server to that site.
If you are auto importing sites and subnets from AD, it's possible the subnet range is already included within a site, which might complicate this a little. There isn't a 'weighting' recommendation as to what package\site server is selected, but I believe the most common method would be speed testing for downloading of the codebases.
So if the remote site server is in that site, the clients should pull from it first, if there is some of the confusion listed above.
So steps
1) Create a new site (Manual)
2) Populate with Subnets
3) Assign Package Server to that site (Manual)