From perusing your code a few times:

Custom event abuse

While it's very cool to have custom events, I would simply remove this extra layer of logic. It does not make sense to call $(networkInfo).trigger('selectNetworkRanges'); if you could just call selectNetworkRanges(). I understand that you would loose access to this but you are accessing networkInfo directly in updateNetworkRangeDropDowns anyway.

DRY (Don't Repeat Yourself)

• In selectNetworkRanges you could do var ranges = this.networkRanges; and then access ranges instead of this.networkRanges every time

• You are building a dropdown in populateSubnetMask and in updateNetworkRangeDropDowns in a different way even though the functionality is very close. With some deep thoughts you could create a helper function that could build a dropdown for both #ddSubnetMask and .mask

• $('.open').click and $('.close').click do the same really, you could just do this:

    $('.close,.open').click(function () {
        $('#drilldowntreecontainer').toggle('1000');
        $('.open').toggle();
    });
  

What's in a name?

• Please avoid short names like r , d

• It is considered good practice to prefix jQuery results with $ so var $dropDown = $('#ddSubnetMask'); for example

Style

• Comma separated variables with a single var are considered better so

    var node = eventArgs.get_node(),
        address = node.get_value().split("/");
  

  instead of

    var node = eventArgs.get_node();
    var address = node.get_value().split("/");
  

• You are using both double quotes and single quotes for your string constants, you should stick single quote string constants. With the possible exception of your data: statements.

Comments

• Great commenting in general, maybe a tad too verbose at times

• You should mention in the top that this code relies on the jQuery Validation Plugin, in fact it would have saved time if you mentioned that in your question ;)

Design

• ddSubnetMask could be set as disabled, you would need a hidden input that contains the actual value to be submitted as per http://stackoverflow.com/a/368834/7602

• The last few functions starting with populateSubnetMask are not within your $(function () {, I would keep it all together

Magic Numbers

• You are commenting that VLAN(type=9), I would still advocate to create a var IS_VLAN = 9 and then use that constant

• Not a magic number per se, 'application/json; charset=utf-8' should be a properly named constant ( it's a DRY issue as well ).

Dancing in the rain

• Your $.ajax calls should deal with error, it will happen at some point

All in all, I could work with this code. You are correct that the code is tightly coupled. I think that's because of the data you have to work with, so I would not worry about it too much.