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giraffedata
join:2009-07-11
San Jose, CA

giraffedata

Member

[Connectivity] How does IP address allocation work?

I have residential service today through a Technicolor TC8305C gateway. I have my own NAT router and wireless access point on my side of the gateway, and am fairly sophisticated with TCP/IP. For various reasons, I am considering having Comcast put the gateway into bridge mode or replacing it with a simple modem.

Question: what will I see from my side of the bridged gateway or modem? I take it there's DHCP service. Do I have to register the MAC address of the thing I plug into the gateway with Comcast for the DHCP server to give it an IP address? What happens if I plug in one machine today and another tomorrow (different MAC addresses)? Will they both get the same IP address? Different IP addresses? One of them won't get an IP address at all?

graysonf
MVM
join:1999-07-16
Fort Lauderdale, FL

graysonf

MVM

[Connectivity] Re: How does IP address allocation work?

Residential single IP address is provided by DHCP.

Router or PC MAC address registration is not required. You will have to provide the modem RF MAC address to Comcast.

The MAC address of your router's WAN interface or that of a single PC with no router will determine the IP assigned by Comcast.

If you change the device for one with a different MAC address, your IP will change. You can not plug in a second device to acquire another public IP address.

If you want multiple public IP addresses you will have to subscribe to routed IP service, use hardware that is acceptable to Comcast, and pay extra for the routed subnet. This may not be available on Residential service though.
AVonGauss
Premium Member
join:2007-11-01
Boynton Beach, FL

AVonGauss

Premium Member

said by graysonf:

If you want multiple public IP addresses you will have to subscribe to routed IP service, use hardware that is acceptable to Comcast, and pay extra for the routed subnet. This may not be available on Residential service though.

A business tier will also still include 5 DHCP addresses I believe and does not require the rental gateway.

train_wreck
slow this bird down
join:2013-10-04
Antioch, TN
Cisco ASA 5506
Cisco DPC3939

train_wreck to graysonf

Member

to graysonf
said by graysonf:

If you want multiple public IP addresses you will have to subscribe to routed IP service, use hardware that is acceptable to Comcast, and pay extra for the routed subnet. This may not be available on Residential service though.

i know in my area it is, you can buy up to 5 IPs on residential, for like ~$14/month
giraffedata
join:2009-07-11
San Jose, CA

giraffedata to graysonf

Member

to graysonf
said by graysonf:

If you change the device for one with a different MAC address, your IP will change. You can not plug in a second device to acquire another public IP address.

Thanks. Can you elaborate on this part? What happens when I plug in the second device? Does DHCP not respond to it because I already have a lease for the other device? Does DHCP give the new device an IP address lease and then Comcast stops routing the prior IP address?

camper
just visiting this planet
Premium Member
join:2010-03-21
Bethel, CT

camper

Premium Member

said by giraffedata:

What happens when I plug in the second device?

 
The DHCP server "sees" a new MAC address from that second device and assigns an IP address to that device. The IP address for that second device is selected from the pool of free IP addresses.
said by giraffedata:

Does DHCP not respond to it because I already have a lease for the other device?

 
The DHCP server will respond to it. The DHCP server will recognize the new device (because of the new MAC address), and respond appropriately. While I've had issues with DHCP servers in the past, I can say that they have never ignored me.
said by giraffedata:

Does DHCP give the new device an IP address lease and then Comcast stops routing the prior IP address?

 
Yes, the new device gets a new IP address lease. (btw, when a new address is assigned, the lease starts out short, and increases over time as it is renewed. imo, a good tactic on Comcast's part.) The old IP address is in sort of a limbo. The address allocation still exists until the lease expires, but Comcast may not consider it to be an active lease. Whether Comcast still routes traffic to that prior IP address, I do not know that answer.
camper

camper to giraffedata

Premium Member

to giraffedata
 
btw, when you say, "What happens when I plug in the second device", I presumed you unplugged the first device and that you have only one device connected to the modem at a time.

If you are speaking of concurrent device connections, then that changes things.

Let's say you have a switch on the home network side of the modem. That switch is plugged into the modem.

If you plug one PC into that switch, then you should get an IP address.

If you plug a second PC into the switch (while the first PC is still active) then you will not get a second IP address.

If you want multiple IP addresses on your home network, then you need to place a router (with a subnetwork and NAT) between the devices requesting the IP addresses and the modem.

NetFixer
From My Cold Dead Hands
Premium Member
join:2004-06-24
The Boro
Netgear CM500
Pace 5268AC
TRENDnet TEW-829DRU

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1 recommendation

NetFixer to giraffedata

Premium Member

to giraffedata
said by giraffedata:

said by graysonf:

If you change the device for one with a different MAC address, your IP will change. You can not plug in a second device to acquire another public IP address.

Thanks. Can you elaborate on this part? What happens when I plug in the second device? Does DHCP not respond to it because I already have a lease for the other device? Does DHCP give the new device an IP address lease and then Comcast stops routing the prior IP address?

If you are talking about a standard Comcast single IP address residential account, it is actually the modem that prevents a second, third, etc device from getting an IP address from Comcast's DHCP server. The modem will will register the first MAC address that is sees, and it will block Internet access for additional devices. This means that if you connect device A and then disconnect it and attach device B, device B will still be ignored until you power cycle or reset the modem (and active simultaneous multiple connected devices are impossible).

If you pay Comcast for additional IP addresses, the situation is similar to the normal single IP address scenario, except the modem will remember up to the maximum number of devices you are paying for instead of only a single device. A business class account is automatically allowed up to 5 simultaneous dynamic IP addresses (if you are using a standard cable modem or a bridged gateway). In some (but not all) locations you can pay for additional IP addresses for a residential account (and the limit is usually only up to 5 IP addresses). This is accomplished in the modem by loading a different config file into the modem for multiple IP address accounts. You should also be aware that some switches may also be considered an attached device by the modem, and that could subtract from the usable attached device count (but most simple unmanaged layer 2 switches are invisible to the modem).

Shown below is the CPE MAC Address table from the SB6121 modem I use on my business class account; perhaps that will clarify things a bit. I currently have three active CPE devices attached (plus a ZyXEL switch that is not visible to the modem), but if I had a standard residential account, the Max CPE count would be 1, and only the first detected device would show up (and only that device would get a DHCP assigned IP address from Comcast).




If you are interested, you can view a diagram of my current network at: »www.dcs-net.net/image/DC ··· gram.gif and that may help explain what you see in the above CPE MAC Address table.