The first method of stopping a DoS attack is simply to drop all traffic related to the target hosts. This is a good tactic for a nonessential protocol, like ICMP (Internet Control Message Protocol), but dropping TCP or UDP (User Datagram Protocol) can impact legitimate traffic, such as HTTP or DNS. However, denying all traffic does keep the attack traffic from impacting the target; thus, in some cases (like a SYN flood), this is better than nothing.
If an attack is originating from one or small number of true hosts, as opposed to being randomly spoofed, a device that tracks source IP addresses will be able to home in on the specific offenders and drop all traffic from those hosts. This will effectively block the attack, earning a perfect mark for this review. However, tracking every unique source IP address is quite a processing feat, requiring large amounts of memory. Therefore, a few of the devices cut corners by dividing the Internet into smaller, more manageable chunks. While this lets the devices track the general origin of an attack, blocking chunks of the Internet -- particularly if they are big chunks or user-dense areas, like cable-modem segments or America Online user proxies -- hurts legitimate traffic. However, this still can be an effective form of attack mitigation.
