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Do 'dumb' switches, i.e., non-smart, non-managed network switches have an IP address?

I'm scanning various ranges and not seeing them show up. I know by their definition they won't have a web interface, but I wonder if they would show up at all?

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    Switches are transparent network devices. – Ron Maupin Feb 25 '18 at 1:22
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    Even smart switches don't necessarily have IP address. I think there used to be switches configurable through RS-232 serial port. – el.pescado Feb 25 '18 at 10:18
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    @Ron They're transparent to forwarded traffic, but that doesn't prevent them from sending and receiving on their own. – grawity Feb 25 '18 at 11:19
  • @el.pescado: Most are still configurable through RS-232, but yes, I've also seen some old ones where console was the only option (no IP support). – grawity Feb 26 '18 at 5:58
54

An unmanaged switch does not even know what IP is. It will forward IP packets as well as many other protocols without ever understanding what the difference is.

All the switch needs to know in order to decide where a packet goes is the MAC address.

Destination and source MAC are two of the three Ethernet header fields which the higher layer must always provide, the third is EtherType which is a 16 bit number indicating what the higher layer protocol is. Some examples are

0x0800 IPv4
0x0806 ARP
0x86DD IPv6

In general these numbers are opaque to switches and are treated as just data. There are exceptions such as 0x8874 and 0x8899 which some switches (usually managed) will use to detect loops.

Most network analysis tools won't be able to tell the difference between a pair of machines directly connected with an Ethernet cable and a pair of machines connected through 1, 2, or more switches.

By probing a switched network from 4 or more machines with carefully crafted source and destination MAC addresses it is possible to deduce some information about the structure of the network by observing if two network paths are using the same CAM table or not. It may also be able to measure roughly the size of the CAM table and how quickly entries are timed out.

Based on such metrics it may be possible for sophisticated network analysis tools to say how many switches are on your network and maybe even make some guesses about the chips used in those switches.

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    Good answer, could be excellent if you added that IP is layer3 and ethernet is layer2 of the OSI model, ARP/MAC are a layer2 addressing system and therefore part of ethernet. – Criggie Feb 25 '18 at 3:06
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    @Criggie I am not going to add that to my answer because it would be misleading. The design of IP does not follow the OSI model. IP is designed to run across many different physical media and does not require a specific number of layers beneath it. As such assigning a layer number to IP is not possible. Moreover ARP is not part of Ethernet. ARP is a protocol running on top of Ethernet. From an Ethernet perspective ARP, IPv4, and IPv6 are three higher layer protocols running in parallel at the same layer just above Ethernet. But I can add a brief explanation of the Ethertype field. – kasperd Feb 25 '18 at 9:49
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    Fair points all. – Criggie Feb 25 '18 at 22:43
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    IP doesn't follow OSI in design, but in practice everyone calls Ethernet L2, IP L3, and TCP/UDP L4. – chrylis Feb 26 '18 at 4:52
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    It might also be worth noting than an unmanaged ethernet switch doesn't even have its own MAC address. – alex.forencich Feb 26 '18 at 6:46
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No, they shouldn't appear at all and they don't have any IP. They're just network switches.

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An IP address is used to access a device. So a switch which has software to allow settings, might need an IP address to let a user access its management system or web interface.

But by definition, a dumb switch is never accessed that way by a user.

It is preprogrammed with a plain "redirect everything" firmware or ASIC (chip). It doesn't need an IP of its own, to be able to move ethernet frames (containing data packets) between incoming and outbound physical ports, and it doesn't have a management system that a user can access.

So it doesn't have (or need, or use) an IP address for itself... and that's why it doesn't have one.

  • At the ethernet level they're frames, or ethernet frames. Not Packets. Sorry for pedantry. – Criggie Feb 25 '18 at 22:44
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    Thanks, its good to be put right, I appreciate the correction and have edited the answer - hopefully now better. I figures that "packets" would be more readily understood. – Stilez Feb 26 '18 at 8:38

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