How can I reference a field of external group to a field of internal group ?

[perunpugno]

I'm using Crystal Report 8.5 and I have a DataBase like this:

Id : primary key
IPaddress : computer ipaddress
Event : general event
Subnet : subnet of IPAdrress

ID IPADDRESS EVENT SUBNET

1 1.2.10.0 A 10 |
2 1.2.10.0 A 10 | 3 (NUMBEREVENTIP) events for 1.2.10.0
3 1.2.10.0 A 10 |
4 1.2.10.1 B 10 |
5 1.2.10.1 A 10 | 2 (NUMBEREVENTIP)events for 1.2.10.1
6 1.2.10.2 C 10 | 1 (NUMBEREVENTIP) event for 1.2.10.2

7 1.2.20.40 A 20
8 1.2.20.40 C 20
9 1.2.20.41 C 20
10 1.2.20.41 C 20
11 1.2.20.41 B 20
12 1.2.20.41 B 20

13 1.2.30.61 A 30
14 1.2.30.61 B 30
15 1.2.30.62 B 30
16 1.2.30.62 B 30
.
.
.

I'd like creating a report which counts "Event" average for every SUBNET like this:


SUBNET ALL_EVENT_AVERAGE

10 2 (3+2+1)/3
20 3 (2+4)/2
30 2 (2+2)/2


I created a group (group1) collecting data by "IPADDRESS" and I added a field in the group1 (NUMBEREVENTIP) which counts every events for IPADDRESS.
I created another group (group2) collecting data by SUBNET but I don't know to create a field in the group 2 (ALL_EVENT_AVERAGE) which references to NUMBEREVENTIP field of group 1.
This field contains average of NUMBEREVENTIP for every subnet.
How can I reference a field of external group to a field of internal group ?