I have not tested this but I think you would have to create a new "IpAdress" metafield and expand the PurchaseOrder meta class with the field. Then make sure the field is filled during checkout.

After that you should be able to make a search more or less like this:

OrderSearchParameters parameters = new OrderSearchParameters();
OrderSearchOptions options = new OrderSearchOptions();

options.RecordsToRetrieve = 100;
options.StartingRecord = 0;
options.Classes.Add("MyMetaClassName");

parameters.SqlMetaWhereClause = string.Format("META.{0} = '127.0.0.1'", "IpAdresssMetaField");

int totalRecords;

var orders = OrderContext.Current.FindPurchaseOrders(parameters, options, out totalRecords);