Why do these overlap?

lon21

I'm trying to do a lab in PT. and get the following error?


LAB_A(config-if)#int se 0/0/0
LAB_A(config-if)#i
LAB_A(config-if)#ip address 172.16.20.1 255.255.0.0
% 172.16.0.0 overlaps with FastEthernet0/0
LAB_A(config-if)#no shutdown
% 172.16.0.0 overlaps with FastEthernet0/0
Serial0/0/0: incorrect IP address assignment
LAB_A(config-if)#do show ip int b
Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 172.16.10.1 YES manual up up

FastEthernet0/1 unassigned YES unset administratively down down

Serial0/0/0 172.16.20.1 YES manual administratively down down

Vlan1 unassigned YES unset administratively down down
LAB_A(config-if)#

jwashington1981

We need more information to be able to answer that.

What's the mask associated with the ip address assigned to Fa 0/0?

mark076h

jwashington1981 wrote: »

We need more information to be able to answer that.

What's the mask associated with the ip address assigned to Fa 0/0?

I don't think so. He is trying trying to put the 172.16 network on both interfaces.

CodeBlox

Your mask is "wrong". In order for this to work, you could use something like a /20 which is 255.255.240.0

A /24 would do too: 255.255.255.0

lon21

jwashington1981 wrote: »

We need more information to be able to answer that.

What's the mask associated with the ip address assigned to Fa 0/0?

I have a Router (Lab A)

Which connects to a pc via fa 0/0 with the ip address of 172.16.10.1
I also have a serial links which connects to a router which should have the ip of 172.16.20.1.

Thanks

lon21

CodeBlox wrote: »

Your mask is "wrong". In order for this to work, you could use something like a /20 which is 255.255.240.0

A /24 would do too: 255.255.255.0

OK this is new.

I'm using a Class B address which has a subnet mask of 255.255.0.0 to me this mean that the first two octets are networks and second two are host.
Therefore only the first two octet should be the same, I can understand that if I was trying to configure 172.16.20.1 and 172.14.30.1 that it would throw up an error. But not this.

jwashington1981

If both ip addresses are in the same subnet, then you cannot do that. Interfaces on a router need to be in each of their own subnets. Otherwise, you would not need to use a router.

lon21

CodeBlox wrote: »

Your mask is "wrong". In order for this to work, you could use something like a /20 which is 255.255.240.0

A /24 would do too: 255.255.255.0

I tried this:

LAB_A(config)#do show ip int b
Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 172.16.10.1 YES manual up up

FastEthernet0/1 unassigned YES unset administratively down down

Serial0/0/0 172.16.20.1 YES manual administratively down down

Vlan1 unassigned YES unset administratively down down
LAB_A(config)#int se 0/0/0
LAB_A(config-if)#ip add
LAB_A(config-if)#ip address 172.16.20.1 255.255.255.0
% 172.16.20.0 overlaps with FastEthernet0/0
LAB_A(config-if)#ip address 172.16.20.1 255.255.240.0
% 172.16.16.0 overlaps with FastEthernet0/0
LAB_A(config-if)#
LAB_A#

lon21

Ok, it worked I just had to configure the other interface with the same subnet.

I'm was just thinking on why is happen, as jwashington1981 said you use a router to route traffic to different networks.

So if I'm using a subnet of 255.255.0.0 then I'm saying that first two octets are for the network and second two are for the host, but if both the interfaces have the same network octets it realises these are on the same network and causes a error.

If I use a subnet as of 255.255.255.0 then the network octets change up two the third octet which is different.

Is my thinking path correct?

Zartanasaurus

lon21 wrote: »

Ok, it worked I just had to configure the other interface with the same subnet.

But why did it happen in the first place, also if you can't use the same subnet on the same router how comes its allowing me to use a /24 on both interfaces on the same router?

Thanks

You can use the same subnet mask.

IE 10.3.80.1 255.255.255.0 and 10.3.81.0 255.255.255.0. They both have the same 255.255.255.0 mask. But the IPs are in different subnets IE 10.3.80.x and 10.3.81.x.

So you can't do 172.16.2.1 255.255.0.0 and 172.16.3.1 255.255.0.0 because both of those IPs are in the 172.16.x.x subnet. If you change them both to 255.255.255.0, then it's usable because 172.16.2 and 172.16.3 don't overlap.

instant000

lon21 wrote: »

Ok, it worked I just had to configure the other interface with the same subnet.

I'm was just thinking on why is happen, as jwashington1981 said you use a router to route traffic to different networks.

So if I'm using a subnet of 255.255.0.0 then I'm saying that first two octets are for the network and second two are for the host, but if both the interfaces have the same network octets it realises these are on the same network and causes a error.

If I use a subnet as of 255.255.255.0 then the network octets change up two the third octet which is different.

Is my thinking path correct?

Maybe?

This is what is important to know.

If you assign a network to one interface, it can't be a "subnet" of another network on your router.

To figure out what I mean by this, work out the following for these two sets of interfaces:
1. subnet address
2. broadcast address
3. range of valid host addresses.

Legal:
e0: 172.16.10.1/25
e1: 172.16.10.129/26

Illegal:
e0: 172.16.10.1/25
e1: 172.16.10.65/26


Note: I used different masks purposely, so you would be able to see that you can assign different masks to interfaces, as long as you make sure they're on separate networks. Being on separate networks is the important thing here. Getting good subnetting skills will make this type of problem easier to understand.

pokerjoker

I am using gns3. I configured on interface f0/0 with ip address 10.3.2.5/24 and tried to configured f1/0 with ip address 10.6.7.1/8 but I am getting overlap message. How can this be when I have used diifferent subnet mask.Please help.

Tyrant1919

10.3.2.5/24 is in the same subnet as 10.6.7.1/8.

10.6.7.1/8 encompasses any 10.x.x.x interface IP, so that cannot be assigned.

Different subnet mask? Yes. But your 10.3.2.5/24 interface is still 'contained' inside the 10.6.7.1/8 subnet.