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CISCO IOS, allow to connect via console, direct RS232 cable to the switch/router, telnet, or sometimes ssh.

* line 0 = Direct RS232 local console via serial cable
* line vty 0 4 = 5 first TCP/IP connection (Telnet or SSH)
* line vty 5 15 = 10 next TCP/IP connection (Telnet or SSH)

You can, for safety disable the telnet, by allowing only SSH via : transport input ssh

This only if it is allowed by your IOS :
CISCO IOS can be powere by a so-called k8 or k9 version which means :
* k9 = Strong AES/3DES encryption
* k8 = Weak DES encryption

When you are running a DHCP server on a CISCO IOS, you can either :

Exclude IPs not to be assigned
Or
Reserved an IP address for a specific host (using MAC address for example)

Here is a configuration example for IP reservation, IP 10.1.2.3 for MAC Address 30.71.ac.bb.ac.cc :

ip dhcp pool MyPoolName
host 10.1.2.3 255.255.255.0
client-identifier 0130.71ac.bbac.cc
default-router 10.1.2.254
dns-server 10.1.2.1 10.1.2.2
domain-name mydomainname.com
client-name wks-resa

In order to check you client-identifier, you can check if it is already bound to a dhcp lease :

show ip dhcp binding

Or run a debug :

debug ip dhcp server packet

If you want to exclude an IP address (e.g server or router) from being assigned by the DHCP server whereas it is in the pool you can use :

ip dhcp 10.1.2.254