Conexao Servlet

Naming exception: javax.naming.NamingException: Unexpected exception resolving reference [Root exception is org.postgresql.util.PSQLException: Connection refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.] Porque ta dando esse erro? (A senha e o login estao certos)

<?xml version="1.0" encoding="UTF-8"?>
<Context>
    <Resource name="jdbc/serverdb"
        factory="org.apache.tomcat.jdbc.pool.DataSourceFactory"
        auth="Container"
        type="javax.sql.DataSource"
        username="ppto"
        password="sdweas"
        driverClassName="org.postgresql.Driver"
        url="jdbc:postgresql://localhost/serverdbtst"
        minIdle="2"
        maxIdle="2"
        maxActive="32"
        logAbandoned="true"
        testWhileIdle="true"
        />
</Context>

public void init() throws ServletException {
        try {
            // lookup initial context.
            Context initCtx = new InitialContext();
            // Look up data source
            this.ds = (DataSource) initCtx.lookup("java:comp/env/jdbc/serverdb");
        } catch(NamingException e){
            throw new ServletException("Naming exception: " + e.toString());
        }
    }

    public void destroy() {
    }

}