  At very-high energies (100 TeV - 1 PeV), the small value of Bjorken-x
($\le10^{-3}-10^{-7}$) at which the parton distribution functions are evaluated
makes the calculation of charm quark production very difficult. The charm quark
has mass ($\sim$1.5$\pm$0.2 GeV) significantly above the $\Lambda$$_{QCD}$
scale ($\sim$200 MeV), and therefore its production is perturbatively
calculable. However, the uncertainty in the data and the calculations cannot
exclude some smaller non-perturbative contribution. To evaluate the prompt
neutrino flux, one needs to know the charm production cross-section in pN ->
c$\bar{c}$ X, and hadronization of charm particles. This contribution briefly
discusses computation of prompt neutrino flux and presents the strongest limit
on prompt neutrino flux from IceCube.
