Lecture5

Access control lists (ACLs) are widely used in computer networking and in network security for mitigating network attacks and controlling network traffic. Administrators use ACLs to define and control classes of traffic on networking devices based on various parameters. These parameters are specific to Layer 2, 3, 4, and 7 of the OSI model.
Virtually any type of traffic can be defined explicitly by using an appropriately Numbered ACL. For example, in the past, the Ethernet type field of an Ethernet frame header was used to define certain types of traffic. An Ethernet type of 0x8035 indicated a reverse address resolution protocol (RARP) frame. Numbered ACLs with a range of 200-299 were used to control traffic according to Ethernet type.
It was also common to create ACLs based on MAC addresses. An ACL numbered 700-799 indi- cates traffic is classified and controlled based on MAC addresses.
After the type of classification is specified, control parameters required for that ACL can be set. For example, an ACL numbered 700-799 could be used to block a client with a specific MAC ad- dress from associating with a predetermined access point.
Today, when classifying traffic, the most common types of parameters used in security-related ACLs involve IPv4 and IPv6 addresses as well as TCP and UDP port numbers. For example, an ACL can permit all users with a specific IP network address to download files from the Internet using secure FTP. That same ACL can be used to deny all IP addresses from traditional FTP access.