Originally Posted by xliioper

IAD-BOS (and reverse -- they are symmetrical domestically) fares start at $74 in BE and $104 in main one-way when connecting via EWR. As others have noted, with any connection over 4 hours domestically (US48) you will have to "break" fare and purchase separate fares on IAD-EWR and EWR-BOS legs. EWR-BOS fares start at $49 in BE and $79 in main. However, IAD-EWR non-stop fares start at $219 in main (there are no BE fares on this route when booking as non-stop). Fare combination restrictions prevent combining BE fares with Main on one ticket, so cheapest ticket with IAD-EWR + EWR-BOS fares (with layover over 4 hours) would $298 one-way.