It is shown that a well-designed lumped $LC$ loop network with a phase inverter can exhibit perfect reflectionless filtering response at its all four ports. The network is composed of two pairs of symmetrical $LC$ ladder circuits, in which one pair is obtained from lowpass transformation at a prescribed frequency based on a special lowpass prototype, and the other pair is from highpass transformation based on the same prototype at the same frequency. The input power, at any port, is divided into the two adjacent ports in the frequency domain through different paths of the loop. Each path between any two ports nearby can be treated as a mono-functional filter, whereas the adjacent two paths work as two filters with complementary responses. The overall circuit performs as a generalized diplexer without a specified “common” or “separation/combination” port. By using a numerical equation solver, the element values of the reflectionless lowpass/highpass prototype filter of order one, three, five, and seven are given. Then, the values of order nine, eleven, thirteen, and fifteen are obtained by using the optimization method. Other filtering functions such as bandpass/bandstop or dual-band can be easily implemented with the proposed prototype by using the standard frequency transformation method. For verification, two sample filters are fabricated and measured. The experimental data agree well with the predictions.