Hola,
Fácil, para squid tienes que seleccionar por qué puerto escuchas y porque puerto respondes, para cada una de las ips que tenga tu servidor
Por ejemplo sería así:
acl tasty3128 myportname 3128 src 24.xxx.210.0/24
http_access allow tasty3128
tcp_outgoing_address 67.xxx.108.128 tasty3128
acl tasty3129 myportname 3129 src 24.xxx.210.0/24
http_access allow tasty3129
tcp_outgoing_address 67.xxx.108.79 tasty3129
acl tasty3130 myportname 3130 src 24.xxx.210.0/24
http_access allow tasty3130
tcp_outgoing_address 67.xxx.108.80 tasty3130
acl tasty3131 myportname 3131 src 24.xxx.210.0/24
http_access allow tasty3131
tcp_outgoing_address 67.xxx.108.221 tasty3131
acl tasty3132 myportname 3132 src 24.xxx.210.0/24
http_access allow tasty3132
tcp_outgoing_address 208.xxx.34.154 tasty3132
acl tasty3133 myportname 3133 src 24.xxx.210.0/24
http_access allow tasty3133
tcp_outgoing_address 208.xxx.34.32 tasty3133