The ornamental design for a modem, as shown and described.
FIG. 1 is a perspective view of a modem showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines immediately adjacent to the claimed areas that define unshaded regions represent unclaimed boundaries while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.