Users obtain a different IP address during each dial-up session
Most ADSL/cable modem ISPs employ DHCP so that 24x7 users don't have a fixed IP address
- But DHCP often results in a fixed address for a long period of time because client will request renewal of same lease
Most attacks today are not directed as a specific address but at any available, vulnerable address. DHCP, then, only makes things a little safer...
In earlier discussion, we mentioned that when your PC or host is attached to the Internet, it has an IP address and is as accessible as any other host on the Internet.
In the dial-up environment, a user receives a different IP address every time they dial up to their ISP. This certainly helps prevent a given host from being targeted by an attacker.
To provide the same protection for 24/7 connections, ADSL and cable modem providers generally employ the Dynamic Host Configuration Protocol (DHCP) to dynamically assign IP addresses to customers. Some service providers purposely terminate the IP address lease frequently; others end up renewing the lease over and over so that a host maintains the same IP address for a long period of time.
Service providers really use DHCP for a couple of reasons, one of which is to help protect users. But of even more significance to the providers is that it makes it difficult for the customer to try to run a server on the premises; if the IP address of the server changes frequently, it becomes hard to advertise and most bona fide companies want a real, permanent host name and IP address to out in the Domain Name System (DNS) database.
Note that DHCP still only helps a little. Most attacks on systems connected to the Internet via dial-up, ADSL, or cable modem are not directed at the victim host; indeed, the host was found by someone trolling for pingable IP addresses within some range of addresses.