hi,
i've a cisco 1800 with .248 pool public ip .The router is connected with dce on serial port to my isp and
is configured with first public ip of my subnet on fe0/0 .
I've to serve to vlan (1 and 20) with this router so i 've connected the router fe0/0 to switch trunk port
and created a subinterface fe0/0.20 with dot1q encryption and ip 192.168.40.1. I also created a dhcp pool for vlan20 interface.
Now i can go to internet trought fe/0.0 . configured vlan 20 device receive 192.168.40.0/24 ip so dhcp pool work.
vlan 20 device can ping 192.168.40.1 and 82.85.162.1 (fe0/0.20 and fe0/0) but not want to go to internet.
show ip nat traslation is empty.

this is my show ip route:

Gateway of last resort is 213.205.53.77 to network 0.0.0.0

     217.133.64.0/32 is subnetted, 1 subnets
C       217.133.64.49 is directly connected, Virtual-Access1
C    192.168.40.0/24 is directly connected, FastEthernet0/0.20
     82.0.0.0/26 is subnetted, 1 subnets
C       82.85.162.0 is directly connected, FastEthernet0/0
     213.205.53.0/32 is subnetted, 1 subnets
C       213.205.53.77 is directly connected, Virtual-Access1
S*   0.0.0.0/0 [1/0] via 213.205.53.77

this is my configuration:
 
Current configuration : 2586 bytes
!
version 12.4
service timestamps debug datetime msec localtime show-timezone
service timestamps log datetime msec localtime show-timezone
service password-encryption
!
hostname ##############
!
boot-start-marker
boot-end-marker
!
logging message-counter syslog
logging buffered 64000
no logging console
enable secret 5 ####################
!
aaa new-model
!
!
!
!
aaa session-id common
clock timezone GMT+1 1
clock summer-time GMT+2 recurring
no ip source-route
!
!
ip dhcp excluded-address 82.85.162.1
ip dhcp excluded-address 192.168.40.1
!
ip dhcp pool LAN_Roma_Eletronica
   network 82.85.162.0 255.255.255.192
   default-router 82.85.162.1
   dns-server 213.205.36.70 213.205.32.70
   lease 0 0 15
ip dhcp pool vlan20
   network 192.168.40.0 255.255.255.0
   default-router 192.168.40.1
   dns-server 8.8.8.8 8.8.4.4
   lease 0 0 15
!
!
ip cef
no ip domain lookup
ip name-server 213.205.32.70
ip name-server 213.205.36.70
multilink bundle-name authenticated
!
!
username ######### password 7 #########
archive
 log config
  hidekeys
!
!
!
!
!
interface FastEthernet0/0
 ip address 82.85.162.1 255.255.255.192
 ip nat outside
 ip virtual-reassembly
 duplex auto
 speed auto
 no keepalive
!
interface FastEthernet0/0.20
 encapsulation dot1Q 20
 ip address 192.168.40.1 255.255.255.0
 ip nat inside
 ip virtual-reassembly
!
interface FastEthernet0/1
 no ip address
 duplex auto
 speed auto
!
interface Serial0/0/0
 bandwidth 2048
no ip address
 encapsulation frame-relay IETF
 no fair-queue
 frame-relay traffic-shaping
 hold-queue 4096 in
 hold-queue 4096 out
!
interface Serial0/0/0.100 point-to-point
 bandwidth 1600
 no cdp enable
 frame-relay interface-dlci 100 ppp Virtual-Template1
  class FR-1600
!
interface Virtual-Template1
 bandwidth 1600
 ip address negotiated
 ip tcp adjust-mss 1410
 keepalive 5
 ppp chap hostname #################
 ppp chap password 7 ################
 ppp pap sent-username ############## password 7 ##############
 ppp ipcp route default
!
ip forward-protocol nd
!
!
no ip http server
ip nat inside source list 110 interface FastEthernet0/0 overload
!
!
map-class frame-relay FR-1600
 frame-relay cir 1600000
 frame-relay bc 200000
 frame-relay mincir 1000000
access-list 1 permit 192.168.40.0 0.0.0.255
access-list 110 permit ip 192.168.40.0 0.0.0.255 any
!
!
control-plane
!
!
line con 0
 session-timeout 60
 exec-timeout 60 0
 privilege level 15
line aux 0
 privilege level 15
line vty 0 4
 session-timeout 60
 access-class 10 in
 exec-timeout 60 0
!
scheduler allocate 20000 1000
end