Components of the Network Model
4-8
8000-A2-GB21-20
November 1997
The following illustration shows the packet flow when the DCE Manager wants to
send a packet to the HotWire 5446 RTU using proxy ARP.
97-15465-02
Local Router
135.1.1.2/
255.255.0.0
135.1.2.1/
255.255.255.0
135.1.3.254
DCE
Manager
135.1.1.1/
255.255.0.0
MCC Card
e1a:135.1.2.2/
255.255.255.0
s1b:135.1.3.1/
255.255.255.0
8546 DSL
Card
s1b:135.1.3.2/
255.255.255.0
5446
RTU
135.1.3.3
Local Router:
ARP Request for RTU
Local Router: Sends packet to
RTU via the MCC and DSL cards
MCC: Proxy ARP (for RTU)
DCE Manager:
Sends packet to
Local Router
In this illustration:
The local router does an ARP request to acquire the HotWire 5446 RTU MAC
address.
The MCC card is in the same network (135.1.3.1) and sees the ARP request.
The MCC card contains a route to the HotWire RTU and knows the RTU is
downstream (generally a host route). The MCC card does an ARP reply for
the HotWire 5446 RTU by responding with its own MAC address.
When the local router receives the ARP reply, it forwards the packet to the
MCC card. The MCC card forwards it to the DSL card which forwards it to the
HotWire 5446 RTU.
For security reasons, a separate management domain is recommended.
However, the management and service domains can share the same subnet.
Separation can be maintained by extending the subnet mask down to the fourth
octet (255.255.255.255).
For example, one management subnet and three service domain subnets could
use the combined subnet mask: 135.1.2.0/255.255.255.0. The management
subnet could be 135.1.2.192/255.255.255.192 and service domain subnets could
be 135.1.2.0/255.255.255.192, 135.1.2.64/255.255.255.192, and
135.1.2.128/255.255.255.192. With these subnet masks, management addresses
use the top quarter of the range (135.1.2.192 through 135.2.2.254) and service
addresses use the lower three-quarters (135.1.2.1 through 135.1.2.191).
