Networking :: How Subnet Masking Works
Nov 15, 2010
I have read NUMEROUS tutorials and explanations on the net about this, but they all seem to assume an informed understanding of IP networking. I have limited knowledge (basically I know how to set up my own home network and use the normal commands for troubleshooting.).I am doing a project at work which requires networking our new store with our original store (throughA VPN).I am just trying to understand in the most basic way how sub netting and subnet masks work. I don't believe this is necessary knowledge for setting up the network via VPN, but I would just like to understand it and I feel like I will be prepared to study further. basically:
1. How does a network mask isolate a particular host on a network?
2. How does changing the mask allow for more addresses to be used?
For example, if my address is, say, 192.168.1.36, how does 255.255.255.0 isolate my machine to receive traffic? I suppose what I am really not understanding is how it does this with more than one host on the network with the same mask.
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Jul 22, 2011
I have TWO L3 + router switch (say switch1 and switch2). I created VLAN100 with VLAN ID 100 in both the switches. I created router 192.168.1.1/24 in Switch1. I created router 192.168.2.1/24 in Switch2. Switch1 is connected with 1.x/24 PCs. PCs are configured with 1.1 gateway. Switch2 is connected with 2.x/24 PCs. PCs are configured with 2.1 gateway. Both Switch1 and switch2 are connected by a trunk to carry VLAN100 data.
1)I have few PCs of 1.x connected to say Switch1 Is it possible for PC with IP 192.168.1.100(x) to ping PC with IP 192.168.2.100(y)?What are the configuration required in both switches to make them communicate ? All the device in both the subnets should ping/communicate with each other.
2)Move PC (192.168.1.100) to switch2. Move PC (192.168.2.100)to switch1.What will happen when PC(1.100) ping (2.100) and vice versa?What will happen when PC(say 1.80 in switch1) pings PC (say 1.100 in switch2) and vice versa? What will happen when PC(say 1.80 in switch1) pings PC (say 2.100 in switch1) and vice versa?
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Jul 4, 2011
I installed Redhat Enterprise linux server5. it has two LAN card and two subnet connected to these two LAN card. i can browse network from these two network easily. But i created VLAN on one network card.Now i cant browse network from these VLAN subnet.
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Jul 19, 2010
Is there any way to disable password masking in Linux - specifically Linux Mint 9?
Ideally I'd like to be able to choose whether to mask or unmask password fields on-the-fly, as is possible when entering passphrases in TrueCrypt.
Sometimes masking is useful (say, on a laptop in public), but often it's totally unnecessary - there's no-one looking at the screen but me, so there's no point to it.
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Mar 15, 2010
I currently have a UNIX script with a function that uses a username and password to connect to the database, retrieve some information and then exit. At the moment, am getting the username and password from a hidden plain text file and permission set to -r--------, i.e. read only to who own the file. The owner of the file is the same owner of the script. At the moment, am not too overly concern as the script works as it is but I want to know if anyone have a suggestion if there is any better way of achieving the same thing with some "form" of security, i.e., for example, masking the username/password.
Basically, I want to be able to mask or hide the username or password in some way. I've thought about encryting the password file, which is in plain text, using simple crypt command from which I retrieve the username and password but I need to decrypt it as well which is sort of similar to how it will be as it is now once it is decrypted. Is there anyway that I can get a username and password in some gibberish format and then translating them into something usable which can be passed on the next command that requires the username/password.
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Jan 8, 2010
I have 2 x PCs and a NAS. Both PCs have 2x NICS. PC connectivity to Internet is via an ADSL router. Current config: Thus far (by choice) I've used static IPs in the 192.168.168.x range for my internal network, connecting all PCs and NAS via a jumbo frame enabled gigabit switch. This has facilitated moving data between the PCs and the NAS at high-speed. As both PCs also required Internet access from time to time, both are also connected to the ADSL router using the 2nd NIC and using subnet 192.168.1.x. I'm sure some of you are shaking your heads by now, but it works well and has been entirely hassle free.
However, I've an app running on the NAS that I'm keen to get Internet connected also. As my existing network devices are not using DHCP I figured the simplest method would be to change my ADSL router configuration such that it is in the same 192.168.168.x subnet, change its DHCP server settings to serve IPs in the same subnet (but in a restricted range I know won't cause any conflicts with the static IPs) and problem solved. On changing the ADSL router confiruration with all machines already booted up and configured as described above, everything worked. All devices could see one another, and access the Internet. On later rebooting the system this no longer works
- Internet access is fine but PCs don't see one another or the NAS. If I disconnect the ADSL Router from the PCs then all devices see one another again.
- Does having 2x NICS on a single device each assigned unique IPs in the same subnet create an issue and can it be overcome? I'd like to overcome it because making one of my PCs the gateway forces me to have it on anytime another device needs access.
- If I'm forced to use Internet connection sharing with one PC on the network connected to the router, how do I best configure this?
- One of the things I need to retain is gigabit connectivity between the PCs and PCs and the NAS (currently achieved by 192.168.168.x subnet being linked via gigabit switch).
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Dec 26, 2010
I am trying to make Apache web server, it's work but cannot be browsed outside my subnet.I am on huge LAN network, not sure how it works, know it have TL-SL2428WEB Smart Switches and probably some kind of DHCP, every user have maximum 64KB, just plug UTP cable, and I have static address on that subnet like: "my.sub.net.ip" witch I know from Java NetworkInteface class or when I try to update IP on my www.dyndns.com Dynamic DNS account with ez-ipupdate, but my subnet is connected to Web with another IP like "my.isp.provider.ip" witch I get from whatismyipaddress.com and I check that is my Cable ISP provider web IP. It is possible that there is more subnet layers between these two IP.Problem occurs when I try to access to my web site from another computer. When I set dyndns host name to be my.sub.net.ip or use localhost everything works fine from my computer, I can access my web site, Apache work, I can use ssh. To illustrate you it work like this: Go to DNS server, find my host name, get my "my.sub.net.ip" and say "This is actually my localhost IP, no need to go on Web, lets loopback". But when I try from another computer even from my college's computer on same subnet it don't succeed. When host name is set on "my.isp.provider.ip" of course nothing works.
We, users of LAN, don't have access to our LAN nor we have admin. For example sometimes we stuck without net and we cannot even press reset button on switches or something like that, we have to wait for day to they, owner of LAN, order some professional to do that. It seems they are just user of Cable TV and IP, and they bay and install LAN without any kind of admin. Also, I use Fedora 13, with httpd, ssh and others packages that comes with Fedora 13 DVD. I know to write bash scripts, use yum, very good in C++ and Java, great programmer, but newbie in networking, , a very little Perl, html, web servers, I heard for DHCP, DNS, NAT, IP forwarding. How you see it is problem in my lack of knowledge about networking. I hope that I succeed to describe by problem with enough details. Please, try to help me. I be very graceful for any kind of help. Don't afraid to bomb my head with any kind of information hard to understand.
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Jan 1, 2011
I've rent a server from a Germany data center,They use a single IP as their gateway that is not in range of my servers IP,Strangly server is working well and when I use 'route -n' command the gateway which is in other subnet appears properly./etc/sysconfig/network contains no gateway IP and I don't know how they set the default gateway while after reboot the gateway is the same, also the IP is static and there's now DHCP.I need to know how they did it so I can do the same on my VPSes.
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Aug 9, 2009
I have 2 NIC's in a box. One of them is external and doesn't matter for this question I don't think.
The other NIC is 192.168.100.3. It hosts an iSCSI Target and SMB Share on my LAN. It's works great.
I have another PC that has NIC at 192.168.100.101 and it hosts my DHCP (Scope: 192.168.100.5-25) server for my LAN.
I have a hardware firewall at 192.168.100.1 and it serves inet to the LAN on a different external connection.
So...
I currently have a WAP (cheap p.o.s. netgear router in WAP mode that keeps overheating). I want to eliminate the WAP device and add a WiFi NIC to the Top PC above.
So, on the first system I would have:
NIC 1: External IP and External Gateway
NIC 2: Static IP 192.168.100.3
Proposed WiFi NIC 3: Static 192.168.100.4
Question:
If I bridge NIC 2 and NIC 3 like this:
# brctl addbr br0
# brctl addif br0 eth0
# brctl addif br0 eth1
Then I simply put the WiFi in Ad Hoc, will another WiFi in Ad Hoc (for example my laptop) be able to "see" 192.168.100.101 (DHCP) & 192.168.100.1 (Gateway) (via WiFI 192.168.100.4 through the bridge to 192.168.100.3 and on to the LAN)?
Also, can the two NIC's be bridge together even though they are the SAME subnet?
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Apr 23, 2010
I installed apache2 on my Ubuntu machine and I am trying to access the server from another subnet. The server is connected using ethernet and has a static ip address. I can ping from the server to any machine in the other subnet but non of the machine on that subnet can ping the server. iptables does not seem to be running
Code:
# service iptables status
iptables: unrecognized service
and its rules are
Code:
# iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain FORWARD (policy ACCEPT)
target prot opt source destination
[code]....
What is preventing my machine from being accessed from the other subnet?
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Feb 3, 2011
This is slackware 12. I'm trying to share my internet connection (on eth0) with a computer connected to eth1.
eth0 is 192.168.0.101, eth1 is 192.168.1.100 (different subnets).
But when I try to make eth1's default gateway the same as eth0's, see the error I get code...
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Aug 19, 2010
I have some trouble setting up a printer on my network. My network is divided so that all wired connections are in one subnet, while all wireless devices are in another. My printer is Canon MP640 and is connected via wireless. When I use the network printer scanner utility from Canon, it will only scan my wired subnet, and is thus unable to see the printer. I can ping the printer, so there is no problem with subnet segregation.
On my router, I have Debian and iptables. My initial thought was that I could somehow set iptables to just forward all packets to an address to my printer. This address, of course, wouldn't exist "physically". But I have no idea whether or not this is the right approach. A suggestion I received from a colleague was to set up forwarding of broadcast packets. However, I am unsure whether this will have an impact on the wired subnet. If any of you could conjure a magical iptables rule for this or have suggestions other than plugging the printer into the wired net
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Jan 8, 2010
I am in a big confusion and searching an answer to calm down my mind.I have my eth0 has following configuration
` ` `
IP 192.168.1.100
mask 255.255.255.0
[code]....
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Mar 25, 2010
I have the zone record 0.16.271.in-addr.arpa which is working fine.
The NS for that zone is setup for my own name server.
I however would like to forward requests for 172.16.0.224/28 to another name server. How would I do this?
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Aug 2, 2010
I'm trying to work out how to route all traffic destined for the internet from all devices connected to eth0 to a wireless router access point via wlan0 on my Slackware box. I also have dhcpd providing ip addresses on the same subnet to any device connected to both eth0 and wlan0.
Diagram:
Code:
____________ ____________
[ ]=============> (wifi) ==============>[ Router / ]
|Workstations| __________ |Access Point| 192.168.2.1
[____________]==>(eth0)==>[ ]<==(wlan0)==>[____________]
192.168.2.253| Server |192.168.2.254
[__________]
If I connect to the router/access point via wireless or directly to the Server via a crossover cable I can obtain an ip address from dhcpd, so that works. As far as I can see I just need to how to route between eth0 and wlan0 then I can provide internet access to those devices!
ifconfig:
Code:
eth0 Link encap:Ethernet HWaddr **:**:**:**:**:**
inet addr:192.168.2.253 Bcast:192.168.2.255 Mask:255.255.255.0
inet6 addr: fe80::201:2eff:fe27:aea3/64 Scope:Link
UP BROADCAST MULTICAST MTU:1500 Metric:1
[code].....
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Mar 1, 2011
Im trying to configure a GRE over IPSec connection between two subnets. The IPSec tunnel is opened and now I want to add a GRE tunnel over it.So, what I didn't understand is why I can't route my subnet over the tunnel, once the only route I have there says that it should route the tunnel IP over the GRE01 interface. Any hint? Thanks.
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Sep 13, 2010
I'm living at a friend's right now, and he's got a wireless access point in the house that I set my laptops wlan0 interface to route through the eth0 to my desktop. It's been working fine for internet sharing and internal networking ( ssh and ftp ) between the laptop and the desktop, but there's a problem with both subnets being able to communicate with each other, and I haven't been able to solve it with DNAT either.
The wireless access point is 192.168.0.1 and has its own lan on 192.168.0.0/24 of which my laptop is 192.168.0.5. I setup the little subnet I created by routing with the laptop to 192.168.1.0/24 and my desktop is 192.168.1.50. With shorewall I can configure iptables to DNAT all of my ssh traffic destined to 192.168.0.5 to 192.168.1.50, but the problem seems to occur when ssh on my desktop fails to connect rather than the DNAT failing.
Using iptraf I've seen that all of the routing does work properly, because I can see on the connection in iptraf that only the SYN packet is being sent from a 192.168.0.x address, there is no ACK packet sent back. I believe this is because in the connection dialog it always shows a 192.168.0.x ip as the source of the connection, but I don't have a route to 192.168.0.0/24 from 192.168.1.0/24 setup and I'm unsure of how to do so.
I'm pretty much in over my head because I don't know what is wrong, I thought it should work like this. Everything else from port configurations, to the configurations of the software itself seems fine so I don't think it's anything like that preventing a connection, but I can't think of what it would be aside from the lack of routing between each subnet.
Is there anyway to just add a route so that 192.168.1.0/24 and 192.168.0.0/24 can communicate with each other directly? I know there should be, I'm just not at all sure how it would be done.
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Jan 22, 2011
does somebody know how dnsmasq / iptables need to be configured such that requests to my public IP from lan are correctly NAT'ed to the host that handles them? Currently my routing device treats them like "oh, these are anyway for me, gnam gnam" which actually doesn't work.Unfortunatly setting up NAT rules that redirect requests from my lan correctly as they are redirected from wan is an option I would like to use only if there is no other possibility.I would like some kind of solution that treats packets that are sent to my public IP as normal packets that are not looped back before they even get out. So they would need to be at least sent to the wan gateway where they are directed back where my firewall can successfully treat them like all other public requests.
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Jul 26, 2011
I just changed my CentOS server from DHCP to static IP address. After the change, I cannot ping other hosts on the same subnet. (I can ping the CentOS itself).The IP address of CentOS is 192.168.0.202.After pinging 192.168.0.106 (106 is on and other host can ping it), arp -a shows? (192.168.0.106) at <incomplete> on eth0 It looks ARP cannot resolve MAC address of hosts 192.168.0.106.
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Jul 17, 2010
Running on Debian Lenny, installed dhcpd with apt-get install dhcp3-server. Machine has two NICs: 192.168.1.1/24 and 10.100.1.17/24.
My /etc/dhcpd.conf:
DHCPDARGS=eth0
subnet 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.100 192.168.1.200;
option routers 192.168.1.1;
[code]....
Trying to start dhcpd reports "no subnet declaration for eth0 (192.168.1.1)" and "no subnet declaration for eth1 (10.100.1.17)". Is dhcpd using /etc/dhcpd.conf, or do I have the wrong config file? If it's right, why is this failing?
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Jul 25, 2011
I need to find myselfe unused IP address in some subnet.
How to determine that particular IP is ununsed in that subnet without asking network admin ?
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Jun 21, 2010
I have a network with multiple subnets from 10.12.056.0 to 10.12.060.0 using net-mask 255.255.248.0 and one gateway 10.12.056.1. I want to setup a dhcp server and wonder how should I config it? All the hosts in the network using the same net-mask and gateway.
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Apr 28, 2010
I have installed a Xen with 2 VM's inside. They are all under the same subnet. The Xen machine can see the outside network but both machines in it cannot.
How do I create the bridge correctly (to xenbr0 i guess) so it fixes this?
Here's what I tried: It's RHEL5.3
1. '/etc/sysconfig/network-scripts/ifcfg-eth0' is ok on both VM's
2. iptables service is disabled, system-config-network shows IP,DNS, GW are correct.
3. On the VM's the route -n shows the gw ip though I cannot ping it.
4. checked hosts.allow, hosts.
5. In '/etc/xen/xend-config.sxp' unmarked:
(network-script network-bridge)
(vif-script vif-bridge)
6. in xen/<images_location>/ I modified vm.cfg so vif = [ 'ip=10.2.0.54' ] for one of my virtual machines.
7. I cannot however find '/etc/network/interfaces', can anyone advise if actually in my case it's the ifcfg-eth0?
8. I basically followed the Bridged Networking scenario in Xen Networking: [URL]
Ifconfig on the Xen Parent:
[root@XEN_PARENT]# ifconfig -a
eth0 Link encap:Ethernet HWaddr 00:50:56:8B:3A:E4
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:130021 errors:0 dropped:0 overruns:0 frame:0
TX packets:75097 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:10315149 (9.8 MiB) TX bytes:12038346 (11.4 MiB)
Interrupt:17 Base address:0x2000 .....
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Feb 14, 2010
I am basically from system side and often confused about the calculation of the IP addresses.Just i want to know that what how can i calculate the following of a IP Address:
(1) Available IP in a Network
(2) Broadcast IP
(3) Network Prefix or Net Mask
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Mar 30, 2009
I have a motherboard which has 4 x 1Gbps Ethernet controllers. I would like to use it as a Gateway for my home network. I have a static IP from my ISP which I can use to configure eth0 (I haven't done it yet as the LE-565 is currently sitting behind my Netgear router until I've got DHCP working). I would like to use eth1, eth2 and eth3 for my LAN. How do I set things up so that DHCP is handing out IP addresses on the same subnet (192.168.0.0/24) on all three interfaces?
P.S. I think what I'm asking is: how do I combine all 3 interfaces to behave like a switch (ie. just like my Netgear router)?
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Jan 12, 2011
My company hosts five Servers with CentOS 5 installed in a Data Center. The Data Center assigns a VLAN to each customer so that they can organize customers and also count traffic. The VLAN assigned to my company is in the form 62.103.X.X with a subnet mask 255.255.255.240 so the IPs that we can use in our Servers are 13. The first IP in the VLAN (for example 62.103.1.1) is the Gateway and the rest, that are calculated from the subnet mask, are assigned to servers(62.103.1.2 up to 62.103.1.14).
The problem I have is that we have run out of IPs in our VLAN. We have used all IPs and we want some more. The Data Center can assign new IPs but not grow our VLAN IPs by expanding the subnet mask. So they gave us a new VLAN with some IPs (for example 62.104.X.X with a subnet mask 255.255.255.192) They told me that I had to use as gateway the gateway of the first VLAN 62.103.1.1 and assign the new IPs as usual. So I assigned to one of the CentOS 5 servers the IP 62.104.1.2 and set the gateway to 62.103.1.1. Unfortunately this did not work and we could not establish connectivity to the Internet.Is it possible to assign a gateway outside of the IPs that are in the subnet? Could it be a problem with the VLAN setup by the Data Center?
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Dec 7, 2009
I have a program that attaches to an interface. I can run two copies of the program on two systems (each running one instance), connect it to a switch, say 1.1.1.1 & 1.1.2.1. In this configuration two instances of the program can communicate and everything is fine.Now, to reduce cost, it want to use only one system with two nics connected to the same switch, running two instances of the above program, each instance attached to two interfaces respectively on the system. I have the following settings:
eth1 Link encap:Ethernet HWaddr XXXXXXXXXXXXXXX
inet addr:1.1.1.1 Bcast:1.1.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
[code]....
However I cannot ping one from the other.
Neither "ping -I eth1 1.1.2.1" nor "ping -I eth2 1.1.1.1" works.
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May 6, 2011
I have a 6to4 tunnel running on Ethernet (subnet 2002:ad4c:16cc:1) without problem. It runs radvd and announces a default route back to the Internet like this: "default via fe80::6a7f:74ff:fe0a:fbec dev br0"
On this same Ethernet I have a Linux plugbox (fe80::225:31ff:fe01:cc) which is a gateway to a network of IPv6 enabled sensors. I've assigned this second subnet 2002:ad4c:16cc:2. How do I get the plugbox to announce "2002:ad4c:16cc:2 via fe80::225:31ff:fe01:cc" so that the hosts on the Ethernet (2002:ad4c:16cc:1) will automatically pick up the route? The route works if I add it to the boxes manually. I've tried getting radvd on the plugbox to do this but I've had no success.
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Jul 28, 2011
I dont even know where to start looking on Google.Just in case this matters, for completeness.I've an inactive eth0 (wired ethernet link) with static IP.Other possibly relevant details:I'm using an old 802.11b card. Cant imagine this is relevant, because the internet is working fine as far as pinging anything outside the local network goes.
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Jan 8, 2011
I'm wanting to set up a network that has a simple screened subnet architecture. The boarder router will have a switch plugged into it. My servers will be plugged into this switch. Also plugged into this switch will be a dedicated firewall. This firewall will protect the internal network. The boarder router is not very flexible. You can have hosts that use dhcp but that get the same internal I.P address each time their interface is configured.Or you can configure static I.Ps on hosts that are outside the dhcp range of the router, but that have the same subnet address.
The most simple way to set up such a network would be for every interface to have a static internal address that is obtained via dhcp from the boarder router. Like i said the boarder router is doesn't give you much freedom. Meaning that it lets you have a static internal address but not a different network address for different interfaces. That is where the confusion is, for me. If all the interfaces that get theirI.P addresses from the boarder router are on the same network, then what about the systems on the internal network. They might as well get their I.Ps from the boarder router as well, to avoid NAT being done twice before their packets get sent out to the Internet?.I wanted a three interface firewall to separate the DMZ from the internal network, instead of using a screened subnet architecture, but the boarder router only allowed one interface to be in the DMZ, i think and it didn't seem to have a well designed way of controlling the DMZ ie setting a new default gateway etc.
Should i let NAT be done twice for packets from the internal network i.e have the dedicated firewall do dhcp and NAT for the internal network, but this firewall's external interface have a static internal I.P from the boarder router, and the internal I.P of the dedicated firewall have a static internal I.P from the router as well?. It looks like all the interfaces have to have the same network and subnet address, except the machines on the internal network(which can get their I.P from the dedicated firewall)
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