  Let $\Omega$ be an algebraic closure of ${\mathbb Q}_p$ and let $F$ be a
finite extension of ${\mathbb Q}_p$ contained in $\Omega$. Given positive
integers $f$ and $e$, the number of extensions $K/F$ contained in $\Omega$ with
residue degree $f$ and ramification index $e$ was computed by Krasner. This
paper is concerned with the number ${\mathfrak I}(F,f,e)$ of $F$-isomorphism
classes of such extensions. We determine ${\mathfrak I}(F,f,e)$ completely when
$p^2\nmid e$ and get partial results when $p^2\parallel e$. When $s$ is large,
${\mathfrak I}({\mathbb Q}_p,f,e)$ is equal to the number of isomorphism
classes of finite commutative chain rings with residue field ${\mathbb
F}_{p^f}$, ramification index $e$, and length $s$.
