Open pipe: antinodes at both ends. $L = n\lambda/2$, so $f_n = nv/(2L)$. Fundamental: $f_1 = v/(2L)$. All harmonics present — same formula as a string fixed at both ends. The open pipe produces a richer sound than a closed pipe because both even and odd harmonics are present. An open pipe of length $L$ has the same fundamental frequency as a string of the same length with the same wave speed.