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Cisco Network part 81

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Tham khảo tài liệu 'cisco network part 81', công nghệ thông tin, quản trị mạng phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | rather than user traffic. Rules of thumb in defining access lists. First define what you want to do and in which directions. An informal drawing is a good first step. As opposed to the usual connectivity drawings among routers it s often convenient to draw unidirectional links between routers. Second informally write out your filtering rules. In general it is best to go from most specific to least specific. Modify the order of writing things to minimize the number of rules needed. Third determine which rules need to be on which routers. Explicitly consider the direction of flow and the possible existence of additional paths that could inadvertently bypass a filter. Can a cisco router be a true firewall This depends on the definition of firewall. Some writers e.g. Gene Spafford in _Practical UNIX Security_ define a firewall as a host on which an inside and or an outside application process run with application-level code linking the two. For example a firewall might provide FTP access to the outside world but it would not also provide direct FTP service to the inside world. To place a file on the FTP external server a designated user would explicitly log onto the FTP server transfer a file to the server and log off. The firewall prevents direct FTP connectivity between the inside and outside networks only indirect applicationlevel connectivity is allowed. Firewalls of this sort are complemented by chokes which filter on network addresses and or port numbers. Cisco routers cannot do application-level control with access control lists. Other authors do not distinguish between chokes and filters. Using the loose definition that a firewall is anything that selectively blocks access from the inside to the outside routers can be firewalls. IP Specific Can the operand field be used with a protocol keyword of IP to filter on protocol ID No. Operand filtering only works for TCP and UDP port numbers. How can I prevent traffic for a certain Internet application to flow in one