随着人们对网络知识的普及，企业或公司的网络安全性，就变得更加重要起来了。关于网络安全的最可靠方法是加装防火墙。
在这里我讲述一下关于加装防火墙前后的路由配置变化，因为在原先没有防火墙的情况下，路由既起到路由选择的作用，又起到网关的作用。当加装防火墙的后，局域网的网关就设为防火墙的局域网IP地址。要修改路由首先还是先看该网络的拓扑结构。在这里我所描述的是这样拓扑结构：
图 1
一、先将进入路由器设置将原来的配置备份一份，虽然这一份备份以后不一定用的上，可是万一防火墙安装失败呢？
图 2
下面为没有安装防火墙以前的路由器配置情况。
User Access Verification
Password: （键入TELNET密码，如果你是直接用CONSOLE口进入没有此项提示）
Router>en
Password:
Router#show config （察看ROUTER配置情况命令）
Using 810 out of 7506 bytes
!
version 12.1
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Router （ROUTER名字，这里为默认名字ROUTER）
!
enable secret 5 $1$FreK$4oQGtvDEF1jv8dh3NNXnN0.
enable password 123456789 （特权密码，当然这是加密的）
!
ip subnet-zero
!
interface Ethernet0 (配置局域网e0口)
ip address 192.168.1.1 255.255.255.0 （e0口在其局域网中对应的ip为192.168.1.1
ip nat inside 255.255.255.0是表示为C类网络)
!
interface Ethernet1 (E1口没有激活,也没有配置)
no ip address
shutdown
!
interface Serial0
bandwidth 2048
ip address 211.97.213.41 255.255.255.252 (此为定义ROUTER外部接口的IP
ip nat outside 255.255.255.252表示此合法的INTERNET-IP)
encapsulation ppp
!
ip nat pool 165 211.97.213.41 211.97.213.46 netmask 255.255.255.248 (isp给你分配的ip)
ip nat inside source list 1 pool 165 overload
ip classless
ip route 0.0.0.0 0.0.0.0 Serial0
no ip http server
!
access-list 1 permit 192.168.1.0 0.0.0.255
!
line con 0
transport input none
line vty 0 1
password 123456
login
!
end
Router#
二、按照图1装上防火墙。
将从路由器到交换机上的线，改为先从路由器到防火墙，然后用防火墙的E0口接交换机。
图 3
进入路由器配置模式修改，将路由器的配置改为：
Using 942 out of 7506 bytes
!
version 12.1
service timestamps debug uptime
service timestamps log uptime
service password-encryption
!
hostname router
!enable secret 5 $1$FreK$4oQGtvDEF1jv8dh3NNXnN0
enable password 123455676!
!
ip subnet-zero
!
crypto ipsec transform-set test esp-des esp-md5-hmac
!
crypto map vpnmap 1 ipsec-isakmp
! Incomplete
set transform-set test
match address 100
interface Ethernet0
ip address 211.97.213.41 255.255.255.248
interface Ethernet1
no ip address
ip nat inside
no ip route-cache
no ip mroute-cache
shutdown
!
!
interface Serial0
description internet
bandwidth 2048
ip address 211.97.209.145 255.255.255.252
ip nat outside
encapsulation ppp
no ip route-cache
no ip mroute-cache
!
ip classless
ip route 0.0.0.0 0.0.0.0 Serial0
ip http server
!
route-map nonat permit 10
match ip address 110
!
!
line con 0
transport input none
line vty 0 4
password 123456
login
!
end
三、这时候，你可以配置你的防火墙了，以下是防火墙的配置情况：
PIX Version 5.1(2)
nameif ethernet0 outside security0
nameif ethernet1 inside security100
nameif ethernet2 pix/intf2 security10
hostname imrac_c_pix
fixup protocol ftp 21
fixup protocol http 80
fixup protocol h323 1720
fixup protocol rsh 514
fixup protocol smtp 25
fixup protocol sqlnet 1521
no names
access-list 100 permit ip 192.168.1.1 255.255.255.0 192.168.0.0 255.255.255.0
access-list 100 permit ip 192.168.1.1 255.255.255.0 192.100.0.0 255.255.255.0
pager lines 24
logging on
no logging timestamp
no logging standby
no logging console
no logging monitor
no logging buffered
no logging trap
logging facility 20
logging queue 512
interface ethernet0 auto
interface ethernet1 auto
interface ethernet2 auto shutdown
mtu outside 1500
mtu inside 1500
mtu pix/intf2 1500
ip address outside 211.97.213.44 255.255.255.248
ip address inside 192.168.1.1 255.255.255.0
ip address pix/intf2 127.0.0.1 255.255.255.255
no failover
failover timeout 0:00:00
failover ip address outside 0.0.0.0
failover ip address inside 0.0.0.0
failover ip address pix/intf2 0.0.0.0
arp timeout 14400
global (outside) 1 211.97.213.45 netmask 255.255.255.248
nat (inside) 0 access-list 100
nat (inside) 1 0.0.0.0 0.0.0.0 0 0
route outside 0.0.0.0 0.0.0.0 211.97.213.41 1
timeout xlate 3:00:00 conn 1:00:00 half-closed 0:10:00 udp 0:02:00
timeout rpc 0:10:00 h323 0:05:00
aaa-server RADIUS protocol radius
no snmp-server location
no snmp-server contact
snmp-server community public
no snmp-server enable traps
floodguard enable
sysopt connection permit-ipsec
crypto ipsec transform-set trans esp-des esp-md5-hmac
crypto map vpnmap 40 ipsec-isakmp
crypto map vpnmap 40 match address 100
crypto map vpnmap 40 set transform-set trans
crypto map vpnmap interface outside
isakmp enable outside
isakmp identity address
isakmp policy 1 authentication pre-share
isakmp policy 1 encryption des
isakmp policy 1 hash md5
isakmp policy 1 group 1
isakmp policy 1 lifetime 86400
telnet 192.168.1.88 255.255.255.255 inside
telnet timeout 5
terminal width 80
Cryptochecksum:7fd10854228b7e32b2808508f49a65a7
