Feistel networks were broken in DES but not triple DES. Some final AES candidates not approved also used Feistel networks $2^{36}$ plain text attacks. Reduction of $2^{16}$ possible keys for single DES: $4^{48/6} = 4^{8} = 2^{16}$.

First for a one round Feistel network: $R_0$ and $f (R_O, k_1) = R_1 \oplus L_0$, $k_1$ becomes known.

For two round Fiestel: Use the same method twice: $$(R_0, f(R_0,l_1)=L_2 \oplus L_0) \ (L_2,f(L_2,K_2)= R_2 \oplus R_0)$$ 

A nice reference would be [CryptoLecture 04](http://www.icg.isy.liu.se/courses/tsit03/forelasningar/cryptolecture04.pdf) (PDF) from Daniel Boneh's crypto course on Coursera.