354 Chapter 11: Scaling IP Addresses with NAT
Turning PAT on is a system-wide command to the 700 series router. The definition for the port-
handler function is done within a profile. There are a number of limitations that must be
addressed while using this technology:
·
ping from an outside host ends at the router. Hence, end-to-end connectivity testing is not
possible.
·
Only one inside web server, FTP server, Telnet server and so forth is supported because
all port traffic is defined by a single ip porthandler command.
·
Only 15 port handlers are supported in a single configuration.
The limitations specified should not be a deterrent to the use of PAT on a 700 series router. You
should remember the market positioning of this device and realize that small remote offices can
take advantage of the translation function to share resources on a larger network.
The two commands associated with PAT on the 700 series router are set ip pat on, which is a
global command that requires no arguments, and set ip pat porthandler, which has the
following arguments:
default
| telnet | ftp | smtp | wins | http | port ip-address | off
The telnet, ftp, smtp, wins, and http arguments declare the well-known ports for those
protocols. The key arguments are default and port ip-address. The default argument specifies
any port that is not declared by another (there are up to 15) set ip pat porthandler command.
The port ip-address is used when the administrator must specify a port other than the defined
ports--Telnet, FTP, SMTP, WINS, and HTTP.
The limiting numbers for the 700 series router are as follows:
·
400 PAT entries are allocated for sharing among the inside machines.
·
Only 15 port handler addresses can be used.
·
1500 maximum MAC addresses can be supported.
The bottom line is that the 700 series router can be configured for a lot more than a 128-kbps
ISDN line can handle. The limitation is not what the device can do, but what can be done on the
resource that the device uses.