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Page 96 / 236 Scroll up to view Page 91 - 95
Chapter 11 DHCP Server
EMG2926-Q10A User’s Guide
96
11.4
DHCP Client List Screen
The DHCP table shows current DHCP client information (including IP Address, Host Name and MAC
Address) of network clients using the EMG2926-Q10A’s DHCP servers.
Configure this screen to always assign an IP address to a MAC address (and host name). Click
Network > DHCP Server > Client List
.
Note: You can also view a read-only client list by clicking
Monitor > DHCP Server
.
Figure 54
Network > DHCP Server > Client List
The following table describes the labels in this screen.
First DNS Server
Second DNS Server
Third DNS Server
Select
Obtained From ISP
if your ISP dynamically assigns DNS server information
(and the EMG2926-Q10A's WAN IP address). The field to the right displays the (read-
only) DNS server IP address that the ISP assigns.
Select
User-Defined
if you have the IP address of a DNS server. Enter the DNS server's
IP address in the field to the right.
Select
DNS Relay
to have the EMG2926-Q10A act as a DNS proxy. The EMG2926-
Q10A's LAN IP address displays in the field to the right (read-only). The EMG2926-Q10A
tells the DHCP clients on the LAN that the EMG2926-Q10A itself is the DNS server. When
a computer on the LAN sends a DNS query to the EMG2926-Q10A, the EMG2926-Q10A
forwards the query to the EMG2926-Q10A's system DNS server (configured in the
WAN
screen) and relays the response back to the computer. You can only select
DNS Relay
for one of the three servers; if you select
DNS Relay
for a second or third DNS server,
that choice changes to
None
after you click
Apply
.
Select
None
if you do not want to configure DNS servers. If you do not configure a DNS
server, you must know the IP address of a computer in order to access it.
Apply
Click
Apply
to save your changes back to the EMG2926-Q10A.
Cancel
Click
Cancel
to begin configuring this screen afresh.
Table 35
Network > DHCP Server > Advanced (continued)
LABEL
DESCRIPTION
Table 36
Network > DHCP Server > Client List
LABEL
DESCRIPTION
#
This is the index number of the host computer.
Status
This field displays whether the connection to the host computer is up (a yellow bulb) or
down (a gray bulb).
Host Name
This field displays the computer host name.
IP Address
This field displays the IP address relative to the # field listed above.
Page 97 / 236
Chapter 11 DHCP Server
EMG2926-Q10A User’s Guide
97
MAC Address
This field shows the MAC address of the computer with the name in the
Host Name
field.
Every Ethernet device has a unique MAC (Media Access Control) address which uniquely
identifies a device. The MAC address is assigned at the factory and consists of six pairs of
hexadecimal characters, for example, 00:A0:C5:00:00:02.
Reserve
Select this if you want to reserve the IP address for this specific MAC address.
Apply
Click
Apply
to save your changes back to the EMG2926-Q10A.
Cancel
Click
Cancel
to reload the previous configuration for this screen.
Table 36
Network > DHCP Server > Client List (continued)
LABEL
DESCRIPTION
Page 98 / 236
EMG2926-Q10A User’s Guide
98
C
HAPTER
12
NAT
12.1
Overview
NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address of a host in
a packet. For example, the source address of an outgoing packet, used within one network is
changed to a different IP address known within another network.
The figure below is a simple illustration of a NAT network. You want to assign ports 21-25 to one
FTP, Telnet and SMTP server (
A
in the example), port 80 to another (
B
in the example) and assign
a default server IP address of 192.168.1.35 to a third (
C
in the example).
You assign the LAN IP addresses to the devices (
A
to
D
) connected to your EMG2926-Q10A. The
ISP assigns the WAN IP address. The NAT network appears as a single host on the Internet. All
traffic coming from
A
to
D
going out to the Internet use the IP address of the EMG2926-Q10A,
which is 192.168.1.1.
Figure 55
NAT Example
This chapter discusses how to configure NAT on the EMG2926-Q10A.
Note: You must create a firewall rule in addition to setting up NAT, to allow traffic from
the WAN to be forwarded through the EMG2926-Q10A.
12.1.1
What You Can Do
•
Use the
General
screen to enable NAT (
Section 12.2 on page 100
).
A: 192.168.1.33
B: 192.168.1.34
C: 192.168.1.35
IP address
192.168.1.1
WAN
LAN
assigned by ISP
FTP, Telnet, SNMP
Port 80
Ports 21 to 25
Page 99 / 236
Chapter 12 NAT
EMG2926-Q10A User’s Guide
99
•
Use the
Port Forwarding
screen to set a default server and change your EMG2926-Q10A’s port
forwarding settings to forward incoming service requests to the server(s) on your local network
(
Section 12.3 on page 101
).
•
Use the
Port Trigger
screen to change your EMG2926-Q10A’s trigger port settings (
Section
12.5.3 on page 106
).
12.1.2
What You Need To Know
The following terms and concepts may help as you read through this chapter.
Inside/Outside
This denotes where a host is located relative to the EMG2926-Q10A, for example, the computers of
your subscribers are the inside hosts, while the web servers on the Internet are the outside hosts.
Global/Local
This denotes the IP address of a host in a packet as the packet traverses a router, for example, the
local address refers to the IP address of a host when the packet is in the local network, while the
global address refers to the IP address of the host when the same packet is traveling in the WAN
side.
Note: Inside/outside refers to the location of a host, while global/local refers to the IP
address of a host used in a packet.
An inside local address (ILA) is the IP address of an inside host in a packet when the packet is still
in the local network, while an inside global address (IGA) is the IP address of the same inside host
when the packet is on the WAN side. The following table summarizes this information.
Note: NAT never changes the IP address (either local or global) of an outside host.
What NAT Does
In the simplest form, NAT changes the source IP address in a packet received from a subscriber
(the inside local address) to another (the inside global address) before forwarding the packet to the
WAN side. When the response comes back, NAT translates the destination address (the inside
global address) back to the inside local address before forwarding it to the original inside host. Note
that the IP address (either local or global) of an outside host is never changed.
The global IP addresses for the inside hosts can be either static or dynamically assigned by the ISP.
In addition, you can designate servers, for example, a web server and a telnet server, on your local
Table 37
NAT Definitions
ITEM
DESCRIPTION
Inside
This refers to the host on the LAN.
Outside
This refers to the host on the WAN.
Local
This refers to the packet address (source or destination) as the packet travels on the LAN.
Global
This refers to the packet address (source or destination) as the packet travels on the WAN.
Page 100 / 236
Chapter 12 NAT
EMG2926-Q10A User’s Guide
100
network and make them accessible to the outside world. If you do not define any servers, NAT
offers the additional benefit of firewall protection. With no servers defined, your EMG2926-Q10A
filters out all incoming inquiries, thus preventing intruders from probing your network. For more
information on IP address translation, refer to
RFC 1631
,
The IP Network Address Translator (NAT)
.
How NAT Works
Each packet has two addresses – a source address and a destination address. For outgoing packets,
the ILA (Inside Local Address) is the source address on the LAN, and the IGA (Inside Global
Address) is the source address on the WAN. For incoming packets, the ILA is the destination
address on the LAN, and the IGA is the destination address on the WAN. NAT maps private (local)
IP addresses to globally unique ones required for communication with hosts on other networks. It
replaces the original IP source address in each packet and then forwards it to the Internet. The
EMG2926-Q10A keeps track of the original addresses and port numbers so incoming reply packets
can have their original values restored. The following figure illustrates this.
Figure 56
How NAT Works
12.2
General
Use this screen to enable NAT and set a default server. Click
Network > NAT
to open the
General
screen.
Figure 57
Network > NAT > General

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