  Let $k = \mathbb{F}_{q}(T)$ be the rational function field over a finite
field $\mathbb{F}_{q}$, where $q$ is a power of $2$. In this paper we solve the
problem of averaging the quadratic $L$-functions $L(s, \chi_{u})$ over
fundamental discriminants. Any separable quadratic extension $K$ of $k$ is of
the form $K = k(x_{u})$, where $x_{u}$ is a zero of $X^2+X+u=0$ for some $u\in
k$. We characterize the family $\mathcal I$ (resp. $\mathcal F$, $\mathcal F'$)
of rational functions $u\in k$ such that any separable quadratic extension $K$
of $k$ in which the infinite prime $\infty = (1/T)$ of $k$ ramifies (resp.
splits, is inert) can be written as $K = k(x_{u})$ with a unique $u\in\mathcal
I$ (resp. $u\in\mathcal F$, $u\in\mathcal F'$). For almost all $s\in\mathbb C$
with ${\rm Re}(s)\ge \frac{1}2$, we obtain the asymptotic formulas for the
summation of $L(s,\chi_{u})$ over all $k(x_{u})$ with $u\in \mathcal I$, all
$k(x_{u})$ with $u\in \mathcal F$ or all $k(x_{u})$ with $u\in \mathcal F'$ of
given genus. As applications, we obtain the asymptotic mean value formulas of
$L$-functions at $s=\frac{1}2$ and $s=1$ and the asymptotic mean value formulas
of the class number $h_{u}$ or the class number times regulator $h_{u} R_{u}$.
