Why do I need an ARP entry?

Discussion in 'Computing and Networks' started by KeepItSimpleStupid, Dec 28, 2015.

  1. KeepItSimpleStupid

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Interesting issue:

    I have been running WIFI with no issues in seeing my HP printer, a 2015dn. The printer is on the wired LAN. There is a wireless b/g access point and 3 repeaters. One does b/g. The other two repeaters (ASUS RT-AC68u) do wireless b/g/n/ac and create another SSID for the 5G network,

    When using the wireless network from my laptop, I was able to see the printer from everywhere. When I plugged into the b/g repeater, I saw the printer. When I plugged into one of the ASUS repeaters, I was unable to see the printer without adding an ARP entry.

    I did not try the other repeater, nor did I try adding the ARP entry temporarily on the ASUS repeater.

    Wireless crapped out on my laptop, so I went wired through the repeaters.

    Why did I need the ARP entry?
     
  2. sailorjoe

    Member

    Jun 4, 2013
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    https://en.m.wikipedia.org/wiki/Address_Resolution_Protocol
    If I understand your set up, your two ASUS repeaters are also acting as Access Points to create your 5G SSID. That probably where your computer got confused. Was it connecting to the repeater or the new SSID?
    Adding the ARP entry forces the two local wireless networks to talk to each other.
    Anyway, that's my guess from here. I have a,friend who described a similar situation recently, and that was the cause.
     
    djsfantasi likes this.
  3. KeepItSimpleStupid

    Thread Starter Well-Known Member

    Mar 4, 2014
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    The set-up is really weird (not in any particular order). I actually have a snow Apple Airport router with a broken wireless card in it. At the time, it was the only way to have wireless dial-up Internet. The dial-up internet was free. DSL wasn't available yet. I added an Belkin router configured as an access point to get a b/g access point and "fix" the wireless card.

    When I got DSL, I put in a filter and used a power line modem to get the WAN connection from near the NID to the center of the house. That connection was replaced with a wire after two failures of the powerline ethernet boxes. Eventually, that link was replaced with a wire.

    There was always one spot where I had difficulty getting wireless service and I added a b repeater from D-Link. Eventually, it was replaced by a DWL-2100AP b/g because a laptop had trouble connecting to a b type repeater.
    So, this guy repeats b/g/n SSID#1 without a physical wired connection.

    When I plug into this and attempt to add a printer, it sees another wired device that could act as a ethernet to US printserver. I'm not using it as such. Interestly, the odd device doesn't appear when you connect wirelessly. It does appear when you connect to the wired port on the repeater though.

    So, the plan WAS to use the ASUS as a router and another ASUS as a repeater,and not running the stock firmware.

    That plan, sort of, fell through until I had plenty of time to make the change. That never happened. I tried to add wireless AC to a Linux laptop and that ended up with no support. Luckily I tried to add the internal bluetooth/wireless b/g/n/ac card externally first.

    The antennas of the ASUS suck. They break too easily and they are too long. I fixed that. Meanwhile, I figured out how to mount DIN rail clips to the ASUS, so it can be mounted between the rafters in the basement.

    I also attempted to clean up the infrastructure and added a RAID server, So, a mostly successful attempt was made to add a patch panel and clean up power distribution end Ethernet routing. The wiring was re-routed and cleaned up significantly. A 48 port network patch panel was installed. Outlets exist in a few areas, but they are not complete. The patch panel is essentially unwired. An upgrade of telco is planned too.

    There's a 10/100 and gigabit switch in this location due at some point to be replaced. Wires do run to a wired laser printer and slingbox. I eventually want to replace a wireless color printer with a wired connection and run a phone line there for the FAX. The laser printer is in the same room too. So, there is just a bunch of interconnects to a skype gateway, switches, routers, access points etc, and a whole bunch of wall warts. I used a rack shelf and a couple of power squids to deal with the wall warts. A "home automation" system is planned too. A UPS needs to be properly connected to the RAID server.

    So, for fun, I put an ASUS router configured as a repeater there too. I also tried connections to the wired network.
    This router creates another SSID for the 5G signal too. So, two SSID's from a single repeat of the b/g/n SSID.

    I got a couple of toys to play with and there isn't a wired Ethernet connection there yet. One device is a 4K, anyformat TV tuner including IPTV. So, for firmware updates, IPTV and remote control, it needs an ethernet connection. I have the possibility to have 4 there, but they are not wired to the patch. I also got a 4x(ATSC mudulator and IPTV server). These, I decided to connect to the wired connections of another ASUS in repeater mode. It has a gigabit switch, so it's perfect for testing the tuner and modulator. The modulator is too complex.
    As an aside: I did manage to use VLC to stream an IPTV signal to the TV.

    So, as I said, it doesn't matter which wireless network SSID I connect to, I can see the laser printer. If I use the wired connections on the repeaters (essentially wireless to Ethernet) I see different printer devices. An ARP entry is required at the computer to see the laser printer. I found this out when a laptop lost it's wireless capability and I tried to connect it wired to the ASUS above.

    The main purpose of trying to "upgrade" was to provide a "fast path" to the server, but the Laptop only has a 10/100 physical Ethernet connection. It does have a express card slot, so it may be possible to upgrade that to Gigabit. Nearly every path was a dead end.

    Verizon's new DSL router/modem that they wanted to give me as a replacement is relatively stupid. They have 300 mb/s wireless with a 10/100 switch. That doesn't help me when I want to reach my server, does it? I did try running POE to the DSL modem, so it would be covered by the UPS. The UPS broke (magic smoke) and the modem broke. Learning about POE isn't fun either when you don;t have wiring that supports 4 pair. I tried to use a POE injector so I could power the modem through power outages. This is probably due to me not understanding what a splitter does. I still have to verify. the "splitter" can take POE and "Split" 30 W; 15 W to a coaxial power plug and 15 W for POE. Just not sure what happens when the ethernet device is not POE. I would "hope" that the 30 W signature would be given to the PSE and that handshaking would occur separately for the Ethernet side of the PD?

    I also decided that it would be nice to have CLEAR for back-up Internet because you could buy time for hours, days weeks or months as you go. That's now defunct.

    The non-stock firmware (Merlin) supposedly fixes bugs. It also allows you to execute stuff on power up if you wanted like changing the WIFI power level or possibly the ARP entry.

    So when the flow is wireless to wired to WAN the printers are found. When going from (wired to wireless) bridge and then (wireless to LAN) it doesn't work without an ARP entry.
     
  4. sailorjoe

    Member

    Jun 4, 2013
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    63
    Wow! That's an amazing story of perseverance. I find it so much easier to put everything on a wireless network in my home, with dual band capability, and a router that goes to my Comcast modem and out to the Internet. I have one repeater to improve coverage from one end to the other. Wireless printer. Wireless backup disc, but not raid. Then again, all of my devices have wireless capability. If they didn't, I'd use some sort of wireless to Ethernet device to fake it. Or maybe build one! I'm also fortunate to live in a high reliability power grid, near a hospital, so power loss is very rare.
     
  5. eetech00

    Active Member

    Jun 8, 2013
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    Is the wired network and wireless network on the same IP subnet?
    Are they separated by a router?
     
  6. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Do you know the IP addresses and can share them?
     
  7. KeepItSimpleStupid

    Thread Starter Well-Known Member

    Mar 4, 2014
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    OK, today I was connected wirelessly. My other similar computer doesn't see the "other" 5G network at all.
    The one that did just bit the dust.

    Network is generally set up with a long lease fr DHCP, SM of 255.255.255.0 and the 10.0.1.x private network.

    So I did a broadcast to 10.0.1.255 and got back

    .1 Snow Router
    .56 The video server (wired to .207)
    .207 The D-link DWL2100AP repeater
    .220 Belkin F5D7230-5 AP
    .231 Silex SX300GB; USB server

    These devices did not respond:
    .2 (DHCP) RT-AC68U router acting as a repeater 802.11ac
    .3 (DHCP) I'm guessing the Skype gateway
    .4 (DHCP) Me
    .5 (DHCP) A TV tuner (wired to .55)
    .6 (DHCP) Android Phone
    .222 (wired, not DHCP) A RAID server
    .230 (wired) HP Laserjet

    There's another wireless printer with a fixed IP that can connect via the wireless network. Either .225 or .235 Fixed IP hardly used.

    arp entries on the .2 router

    .2
    ASUSWRT RT-AC68U_3.0.0.4 Fri Sep 6 03:45:04 UTC 2013
    admin@RT-AC68U:/tmp/home/root# arp -a
    ? (10.0.1.56) at [ether] on br0
    ? (10.0.1.3) at [ether] on br0
    ? (10.0.1.4) at [ether] on br0
    ? (10.0.1.220) at [ether] on br0
    ? (10.0.1.207) at [ether] on br0
    ? (10.0.1.222) at [ether] on br0
    ? (10.0.1.6) at [ether] on br0
    ? (10.0.1.1) at [ether] on br0
    ? (10.0.1.231) at [ether] on br0

    MAC addresses redacted.

    arp entries at the .55 repeater

    .55
    ASUSWRT RT-AC68U_3.0.0.4 Fri Sep 6 03:45:04 UTC 2013
    admin@RT-AC68U:/tmp/home/root# arp -a
    ? (10.0.1.220) [ether] on br0
    ? (10.0.1.1) at [ether] on br0
    ? (10.0.1.8) at [ether] on br0
    ? (10.0.1.2) at [ether] on br0
    ? (10.0.1.231) at a [ether] on br0
    ? (10.0.1.5) at [ether] on br0
    ? (10.0.1.3) at [ether] on br0
    ? (10.0.1.4) at [ether] on br0
    ? (10.0.1.6) at :7c [ether] on br0
    ? (10.0.1.7) at :59 [ether] on br0
    ? (10.0.1.222) at :88 [ether] on br0
    ? (10.0.1.207) at :de [ether] on br0

    arp entries redacted.

    .7 and .8 is probably the other laptop with a wired and wireless connection.
    I forget where DHCP ends.

    The skype gateway, the server and the printer, gigabit switch go to a 10/100 Hub. The other stuff goes to a gigabit switch.

    The tuner should be on a fixed IP and the server should be on the GB switch.

    BTW: The video server has two Ethernet ports. One for management and one for multicasting/unicasting. Both ports connect to .55

    The wired laptop (Toshiba A665-s6050) that didn't see the printer when connected via a wire to .55 It did see the printer when connected wirelessly.
    It did see two SSID's, not one.

    The Toshiba A665-s6057 only sees one SSID. I didn't try wired.
     
    Last edited: Jan 4, 2016
  8. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Which device(s) is(are) acting as DHCP servers? Could you have a possible conflict, which is causing some devices to be disabled on the network?
     
  9. eetech00

    Active Member

    Jun 8, 2013
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    See if your router(s) have a "proxy arp" function and turn it on.

    Or

    Since you are using one subnet, Make sure your routers/AP are configured to bridge the wired and wireless LANs
     
    Last edited: Jan 4, 2016
  10. fiberstorejames

    New Member

    Aug 5, 2014
    18
    1
    Address Resolution Protocol (ARP) is a protocol for mapping an Internet Protocol address (IP address) to a physical machine address that is recognized in the local network. For example, in IP Version 4, the most common level of IP in use today, an address is 32 bits long. In an Ethernet local area network, however, addresses for attached devices are 48 bits long. (The physical machine address is also known as a Media Access Control or MAC address.) A table, usually called the ARP cache, is used to maintain a correlation between each MAC address and its corresponding IP address. ARP provides the protocol rules for making this correlation and providing address conversion in both directions.

    When an incoming packet destined for a host machine on a particular local area network arrives at a gateway, the gateway asks the ARP program to find a physical host or MAC address that matches the IP address. The ARP program looks in the ARP cache and, if it finds the address, provides it so that the packet can be converted to the right packet length and format and sent to the machine. If no entry is found for the IP address, ARP broadcasts a request packet in a special format to all the machines on the LAN to see if one machine knows that it has that IP address associated with it. A machine that recognizes the IP address as its own returns a reply so indicating. ARP updates the ARP cache for future reference and then sends the packet to the MAC address that replied.

    Since protocol details differ for each type of local area network, there are separate ARP Requests for Comments (RFC) for Ethernet, ATM, Fiber Distributed-Data Interface, HIPPI, and other protocols.
     
  11. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    To add, when a device first connects to the network, it sends a gratuitous ARP, to let other devices add it to their ARP table.
    ARP is the process that allows communication between layers 2&3 of the OSI model. Microsoft's TCP/IP stack post-Vista won't put anything on the wire if an ARP request is not replied to.
     
  12. eetech00

    Active Member

    Jun 8, 2013
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    Since different LAN segments are involved but the there is only one network subnet used, there won't be an arp between segments.
    That's why you have to enable "proxy arp" or add entries to the arp table.

    A network with routers (like the ASUS RT-AC68u) would incorporate multiple subnets. Then manual arp entries wouldn't be needed.

    Since this network is using only one subnet, all segments need to be "bridged" to work with the least amount of administrative effort.
     
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