1

It acts as the "gateway" to the Internet,but also from what i have read,it's the last thing a packet would be sent to while trying to route (when checking the senders' route table).

Maybe it's the last option and packets will be sent there if we have a network divided into sub-networks,with no appropiate record on the table?

Thanks for answering!

  • A default route, e.g. 0.0.0.0/0, in a routing table matches every address. A router with no matching destination will drop traffic, but a router with a default route will forward traffic not matching any other routes toward the default route because it matches everything. – Ron Maupin Feb 4 at 17:06
  • So,it forwards packets to the default route,which is 0.0.0.0/0.What's the meaning behind this address?How can it represent "every address"? – Zach Feb 5 at 5:32
  • The default gateway is the host on the LAN that knows how to forward packets to other LANs. The default gateway probably has a default route, but that is not a requirement. – Ron Maupin Feb 5 at 5:36
1
  • It acts as the "gateway" to the Internet,
  • it's the last thing a packet would be sent to while trying to route

Both definitions are more or less equal; think about the following network:

+-------------------+
| Network           |
| 10.0.1.0/24       |
| 2001:db8:0:1::/64 |
+---------D---------+
          |
+---------A---------+     +------------+
| Network           |     |            |
| 10.0.2.0/24       C-----+  Internet  |
| 2001:db8:0:2::/64 |     |            |
+---------B---------+     +------------+
          |
+---------+---------+
| Network           |
| 10.0.3.0/24       |
| 2001:db8:0:3::/64 |
+-------------------+

Let's say the computer 10.0.2.20 sends some IPv4 packet.

If the destination is in the range 10.0.2.x, the routing table contains the information that the packet can "directly" be sent to the destination.

If the destination is in the ranges 10.0.1.x or 10.0.3.x, the routing table contains the information about the router (gateway) to be used: Either router "A" or router "B".

The computer will first look up all entries in the routing table. If one entry matches (e.g. when sending a packet to 10.0.1.15), the information from that entry will be used.

However, if the destination is in the internet, there is no entry in the routing table. After checking all entries of the routing table, the sending computer will use the default gateway because no entry was matching.

You could also say: If no entry in the routing table was matching, you know that the destination must be in the internet.

Please note that in the subnet 10.0.1.x things are easier: "D" can be the default gateway and the routing table needs not to contain any entries for 10.0.2.x and 10.0.3.x because the router "D" is used both for packets to the internet and for packets to 10.0.2.x and 10.0.3.x.

2

Default routes are used to direct packets addressed to networks not explicitly listed in the routing table. Default routes are invaluable in topologies where learning all the more specific networks is not desirable, as in case of stub networks, or not feasible due to limited system resources such as memory and processing power.

The default gateway is the first Layer 3 device (such as a router or layer 3 switch) on the network to which the switch(es) connects. The switch will forward IP packets with destination IP addresses outside the local network to the default gateway.

In my organization we use ip prefix-list on all our decentralized locations to aggregate the routing table to only the default route. This saves a lot of resources on our equipment (memory/CPU) as the routing table is very large. Only on our main location do we have the full routing table. It also means we can use small routers or layer 3 switches on smaller locations and still have the full potential of our MPLS.

Configuring a Gateway of Last Resort Using IP Commands: https://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/16448-default.html

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