When in 'ed', the prompt is ':'.

Ed has two modes, command mode and insert mode. The insert mode has
no prompt. You exit the insert mode by typing a single '.' on a line.

All commands have the following syntax:

X,Ycmd

or
Xcmd

For example:

1,10p
Will print line 1 to 10.
1,5d
Will delete line 1 to 5.
8p
Will print line 8.
A '.' is the "current line". The current line is the last line
referenced. If you want to print last line + 10 more:
.,.+10p