Squid / IPTables configuration: Dynamic IP Address provided by ISP

I have a network with a broadband connection with dynamic ip address(external ip address provided by the ISP). How do I configure the squid proxy as the isp ip addresses is changing everytime we login. Please click on the following link to view our network diagram http://www.dotweb.in/share-both.jpg

What should be the appropriate IPTABLES commands to enable squid.

Following is our squid.conf file
hierarchy_stoplist cgi-bin ?
acl QUERY urlpath_regex cgi-bin \?
no_cache deny QUERY
hosts_file /etc/hosts
refresh_pattern ^ftp: 1440 20% 10080
refresh_pattern ^gopher: 1440 0% 1440
refresh_pattern . 0 20% 4320
acl all src 0.0.0.0/0.0.0.0
acl manager proto cache_object
acl localhost src 127.0.0.1/255.255.255.255
acl to_localhost dst 127.0.0.0/8
acl purge method PURGE
acl CONNECT method CONNECT
cache_mem 1024 MB
http_access allow manager localhost
http_access deny manager
http_access allow purge localhost
http_access deny purge
http_access deny !Safe_ports
http_access deny CONNECT !SSL_ports
acl lan src 192.168.0.1 192.168.2.0/24
http_access allow localhost
http_access allow lan
http_access deny all
http_reply_access allow all
icp_access allow all
visible_hostname myclient.hostname.com
httpd_accel_host virtual
httpd_accel_port 80
httpd_accel_with_proxy on
httpd_accel_uses_host_header on
coredump_dir /var/spool/squid
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