trigger FieldUpdateTrigger on Contact (after update) {
        Set<Id> contactIds = new set<Id>();
        for(Contact Values : Trigger.new) {
                Contact oldValues = Trigger.oldMap.get(Values.Id);
                if(Values != oldValues) {
                    ContactIds.add(values.id);
                }
        }
        if (!ContactIds.isEmpty()){
        FieldUpdateCallouts.UpdateFieldOnExternalServer(ContactIds);
        }
/*
         Contact Newcon = trigger.new[0];
         Contact Oldcon = trigger.old[0];
         Contact ContactObject = new Contact();
          // This takes all available fields from the required object.
           Schema.SObjectType objType = ContactObject.getSObjectType();
            Map<String, Schema.SObjectField> M = Schema.SObjectType.Lead.fields.getMap();
             for (String str : M.keyset()) {
             try{
                      System.debug('Field name: '+str +'. New value: ' + Newcon.get(str) +'. Old value: '+Oldcon.get(str));
                      if(Newcon.get(str) != Oldcon.get(str))
                      {
                          system.debug('******The value has changed!!!! ');
                          system.debug('******Changed Field Name********** ' +str);
                          Set<Id> allInsertedIds = trigger.newMap.keySet();
                          system.debug('******The Updated Record Id is*****!!!! ' +allInsertedIds);
                      }
                }catch (Exception e)
                 {
                      System.debug('Error: ' + e);
                 }
             }//close for
        // make the asynchronous web service callout here
       // FieldUpdateCallouts.UpdateFieldOnExternalServer();
       */
}

public class FieldUpdateCallouts {
    @future (callout=true)
    public static void UpdateFieldOnExternalServer(Set<Id> ContactIds){
    List<Contact> ConList = new List<Contact>();
    Http http = new Http();

        HttpRequest req = new HttpRequest();
        req.setMethod('POST');
        req.setHeader('Content-Type', 'application/x-www-form-urlencoded');
        String username='';
        String password='';
        Blob headerValue = Blob.valueOf(username +':' +password);
        String authorizationHeader = 'Basic ' + EncodingUtil.base64Encode(headerValue);
        req.setHeader('Authorization', authorizationHeader);
        String targetString ='{"str": "Newcon.get(str)"}';
        String encodedString = EncodingUtil.urlEncode(targetString,'UTF-8');
        req.setBody('encodedString');
        req.setEndpoint('/admin/api');
        HttpResponse res = new HttpResponse();
        res = http.send(req);
}