40 votes
Accepted

What is the difference between 0.0.0.0 and a loopback IP address?

The statement: The IP address 0.0.0.0 [...] means ‘‘this network’’ or ‘‘this host.’’ is misleading. It is not a "or" but "This host on this network." From RFC1122: { 0, 0 } ...
JFL's user avatar
  • 19.6k
14 votes

Why can I send 127.0.0.1 to 127.0.0.0 on my network?

we changed our FW so that the fixed IP addresses are 127.0.0.0 and 127.0.0.1. Generally, that's a really bad idea. It is in violation of RFC 1122: (g) { 127, <any> } ...
Zac67's user avatar
  • 83.9k
13 votes

What is the difference between 0.0.0.0 and a loopback IP address?

If I am correct, a loopback IP address refers to the current host. No. Traffic sent to a loopback address loops back inside the host. You can send traffic to a loopback address as the destination ...
Ron Maupin's user avatar
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8 votes
Accepted

Why can I ping IP addresses from 127.0.0.1 to 127.255.255.254?

The entire address block 127.0.0.0/8 is the block of loopback addresses for a host. There are RFCs that explain this. The goes back at least as far as RFC 990, ASSIGNED NUMBERS: The class A network ...
Ron Maupin's user avatar
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8 votes
Accepted

Which physical interface would be used to send/receive packets intended for a loopback interface?

It depends on routing table of router2 and other devices in your network (if you have any between router1 and router2). Router1 does not influence on it, it can be do something only when packet ...
Konstantin Goncharenko's user avatar
7 votes
Accepted

Routing loopback using OSPF

Loopback interfaces are considered as host routes, so they are always advertised with a /32 mask. If you want to advertise the whole subnet, you have to change the network type to point to point like ...
Ron Trunk's user avatar
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6 votes
Accepted

What's the relationship of a Router with its loopback interface in logic?

Loopback interfaces are virtual interfaces that are considered directly connected interfaces. Your Loopback0 interface is defined with a /32 network, which, by definition, is a local route. How you ...
Ron Maupin's user avatar
  • 99.4k
6 votes
Accepted

IP address that can not be reached from outside of device

Any address in the 127.0.0.0/8 block can never appear anywhere on any network, nor can any address in that block be used as a source or destination address for packets outside the host. The goes back ...
Ron Maupin's user avatar
  • 99.4k
6 votes

Why can I send 127.0.0.1 to 127.0.0.0 on my network?

The way I understand the question is: "How can I make it so our thingamajig DOESN'T work over a network of any kind and only works over a direct cable connection?" Since that is specifically ...
FrameHowitzer's user avatar
5 votes

Loopback Interface speed is 10 Mbps but Incoming and Outgoing Traffic is in Gbps

An answer from a slightly Cisco skewed perspective: Here's a Loopback interface from a Nexus 3164Q, a L3 Switch with 64 QSFP 40G ports and a backplane rated for 5.12Tbit/sec. It has no QoS ...
Marc 'netztier' Luethi's user avatar
5 votes

STP not preventing switching loop

If you enable 'BPDU filtering' on the ports you're discarding the STP packets, thus breaking spanning tree. Either use STP or filter the BPDU's, don't do both.
Teun Vink's user avatar
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5 votes

STP not preventing switching loop

Those are all features of spanning tree. Disable them, study them and decide if you need them. Storm control is just throttling the specified types of traffic like multicast, broadcast. A couple ...
Fixitrod's user avatar
  • 938
5 votes
Accepted

Loopback Interface speed is 10 Mbps but Incoming and Outgoing Traffic is in Gbps

A loopback interface is a logical construct. It doesn't have any physical link speed. Usually, a nominal link speed is indicated for table completeness only, especially when inspected using SNMP or ...
Zac67's user avatar
  • 83.9k
4 votes

What are the most common scenarios in which the 'loopback interface' on a router is used?

A loopback interface is a virtual (i.e. not physical) interface that never goes down, unless you disable it. This way, if you configure an IP on this loopback interface and add it to your OSPF area (...
Benjamin A.'s user avatar
  • 1,687
4 votes

What are the most common scenarios in which the 'loopback interface' on a router is used?

...we learned that every router has a LOOPBACK interface. That is not true. Cisco routers can be configured with loopback interfaces, but they don't have loopback interfaces until you create one. ...
Ron Maupin's user avatar
  • 99.4k
4 votes

Can my default gateway of router be 127.0.0.1?

No. Any address in the 127.0.0.0/8 block can never appear anywhere on any network, nor can any address in that block be used as a source or destination address. The goes back at least as far as RFC ...
Ron Maupin's user avatar
  • 99.4k
4 votes

Why can I send 127.0.0.1 to 127.0.0.0 on my network?

If you don't want your product to work on an IP network, then don't use IP. IPv4 is transmitted using Ethernet packets with an EtherType of 0x800. Wikipedia has a (probably non-exhaustive) list of ...
Keiji's user avatar
  • 141
3 votes
Accepted

Ping loopback without interface IP address

On a point-to-point link, you don't always need an IP address on the router's interface but you must inform the router of this fact, using the ip unnumbered command Setting interface ...
JFL's user avatar
  • 19.6k
3 votes
Accepted

Should the Loopback address be unique across the network?

TL;DR Be very careful with that idea, unless you know EXACTLY what you are doing. Longer: A) Adresses are in VRFs It will cause no harm if these sets of loopback interfaces are on PE routers (or ...
Marc 'netztier' Luethi's user avatar
3 votes
Accepted

BGP update source basics

To expand on @cown 's answer, the reason this works is because King initiated the connection, sending an open request to 2.2.2.2. Kong received the request on that interface, so it responded from ...
Ron Trunk's user avatar
  • 67.3k
3 votes
Accepted

How is harpinning (NAT loopback) used in practice?

Nat loopback is useful when you have some DNS issue. Imagine that you host your own email server inside the network. You need to access it when you are in the office and when you are outside. Your ...
JFL's user avatar
  • 19.6k
3 votes

Can Loopback IP be Public

Yes a loopback IP can be and often is a public IP. However you are unlikely to have much luck propagating a route for a single IP outside of your own network. The de-facto policy on the internet is ...
Peter Green's user avatar
  • 13.3k
3 votes

RIP routes are not being advertised

You need to include the link addresses (172.33.x.x) in the rip network statements. Otherwise, RIP won't send advertisements out those interfaces.
Ron Trunk's user avatar
  • 67.3k
3 votes

Why can I send 127.0.0.1 to 127.0.0.0 on my network?

Switches don't look at the IP address when forwarding frames. You can use whatever IP address you want, as long as the MAC addresses are correct your switch will happily forward it. It doesn't matter ...
RobinG's user avatar
  • 1,969
3 votes

Why can I send 127.0.0.1 to 127.0.0.0 on my network?

Here are some ideas. None guaranteed. You didn't specify how important it is that someone can't deliberately access the packets. Use a weird connector and don't sell adapter cables compatible with ...
user253751's user avatar
2 votes

Whats's the function of a loopback interface in routing protocols?

Adding to @Ron Maupin great answer I would further say that the (wise) choice of the router ID to be the loop back interface will be more "powerful" upon link failure scenarios. As others mentioned, ...
Guy Avraham's user avatar
2 votes

Setting a loopback/management address through a layer 2 trunk

If you are set on using a loopback address to manage the device, then you will need to add two IP addresses - one in-band l3-interface attached to the VLAN, and one on the loopback interface. Your ...
Benjamin Dale's user avatar
2 votes

STP not preventing switching loop

In addition to the previous answers from Zac67 and Teun Vink, you must ensure that all switches in your network are configured with the same version of STP (STP / RSTP / MSTP /PVSTP), and set the STP ...
JFL's user avatar
  • 19.6k
2 votes

STP not preventing switching loop

In addition to the essential problem - filtering BPDUs - you might want to select RSTP or MSTP (whatever your switch supports) to make STP bring up the links faster.
Zac67's user avatar
  • 83.9k

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