Hi
To reserve some bandwidth for the single ip would suggest to try out this .
1.Check out the bandwidth command configured under the interface which will be used for going out.
It has to be the same with respect to your connectivity speed (i.e., 2048 if its an E1 link)
2.Create an ACL permitting that specific ip in source and destination as any.
if you want to enable the gurantee between with any particular host then do configure the remote node in the destination point.
3.Create a class-map in the global config mode and using access-group bind the access list under that.
4.Create a policy-map and under the same configure the class-map which you have just created.
Under the same configure the bandwidth which you want to allocate for this class of traffic which is 50K as per your requirement here.
5.finally bind the policy map under your WAN(serial) interface on the out direction.
but do remember that this qos config has only local significance,if you want to have the kinda of gurantee or reservation throughout the path in which the pakcet traversee then you need to have similar kinda configs in place ,so that you can have desired results..
would suggest to have look on this link for more help and clarity in configuring the same..
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat4000/12_18a/config/qos.htm
regds
