Netopia 4000-Series manual Parts of a filter, Port numbers, Aurp AppleTalk 387, Who 513

Security 10-21

Parts of a filter

A filter consists of criteria based on packet attributes. A typical filter can match a packet on any one of the following attributes:

■The source IP address (where the packet was sent from)

■The destination IP address (where the packet is going)

■The type of higher-layer Internet protocol the packet is carrying, such as TCP or UDP

Port numbers

A filter can also match a packet’s port number attributes, but only if the filter’s protocol type is set to TCP or UDP, since only those protocols use port numbers. The filter can be configured to match the following:

■The source port number (the port on the sending host that originated the packet)

■The destination port number (the port on the receiving host that the packet is destined for)

By matching on a port number, a filter can be applied to selected TCP or UDP services, such as Telnet, FTP, and World Wide Web. The following tables show a few common services and their associated port numbers:

Internet service	TCP port	Internet service	TCP port
FTP	20/21	Finger	79
Telnet	23	World Wide Web	80
SMTP (mail)	25	News	144
Gopher	70	rlogin	513
Internet service	UDP port	Internet service	UDP port
Who Is	43	AppleTalk Routing	202
		Maintenance (at-rtmp)
World Wide Web	80	AppleTalk Name Binding	202
		(at-nbp)
SNMP	161	AURP (AppleTalk)	387
TFTP	69	who	513