Line VTY 0 4 - where are they?

in CCNA & CCENT
I'm studying CCNA chapter one. One task is configure vty to have password security. The book command is "#line vty 0 4". But I read online vty can actually go up to 15 ofr a total of 16 VTY lines. So should I type "#line vty 0 16"?
Somehow I figured out how to look up how many vty ports my router has by typing "show line". My Cisco1721 didn't even have vty 0 to 4. It has vty 6 to 10. What's going on? Can you guys explain?
R1#show line
Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int
* 0 CTY - - - - - 0 0 0/0 -
1 TTY - inout - - - 0 4 0/0 -
5 AUX 9600/9600 - - - - - 0 0 0/0 -
6 VTY - - - - - 0 0 0/0 -
7 VTY - - - - - 0 0 0/0 -
8 VTY - - - - - 0 0 0/0 -
9 VTY - - - - - 0 0 0/0 -
10 VTY - - - - - 0 0 0/0 -
Line(s) not in async mode -or- with no hardware support:
2-4
Somehow I figured out how to look up how many vty ports my router has by typing "show line". My Cisco1721 didn't even have vty 0 to 4. It has vty 6 to 10. What's going on? Can you guys explain?
R1#show line
Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int
* 0 CTY - - - - - 0 0 0/0 -
1 TTY - inout - - - 0 4 0/0 -
5 AUX 9600/9600 - - - - - 0 0 0/0 -
6 VTY - - - - - 0 0 0/0 -
7 VTY - - - - - 0 0 0/0 -
8 VTY - - - - - 0 0 0/0 -
9 VTY - - - - - 0 0 0/0 -
10 VTY - - - - - 0 0 0/0 -
Line(s) not in async mode -or- with no hardware support:
2-4
Comments
Think about the command you typed. "Line VTY 0 16". Start at VTY0 and count up 16 lines. Found your error yet?
Start by configuring telnet access likes 0-4 and no exec to the rest and you'll see how it works.
router(config)#line vty 0 ?
hth
Hi, I tried your command. It shows
R1(config)#line vty 0 ?
<1-15> Last Line number
<cr>
So it has 16 vty lines. If I only set up password on line vty 0 4. How does a hacker hack into a Cisco router? Because when I telnet into a Cisco router, it doesn't ask me which vty line to use.