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New Member

IP Subnetting question

Are these wrong vlan IP ranges or are they correct:

1st vlan id: 10.4.0.0 /20

• 1st valid IP address: 10.4.0.1 /20

• Last valid Ip address: 10.4.15.254 /20

• Broadcast: 10.4.15.255 /20

2nd vlan id: 10.4.16.0 /20

• 1st valid IP address: 10.4.16.1 /20

• Last valid Ip address: 10.4.31.254 /20

• Broadcast: 10.4.31.255 /20

3rd vlan id: 10.4.32.0 /20

And so on. . .

Could I have done the following as well?

1st vlan id: 10.0.0.0 /20

• 1st valid IP address: 10.0.0.1 /20

• Last valid Ip address: 10.0.15.254 /20

• Broadcast: 10.0.15.255 /20

2nd vlan id: 10.4.16.0

• 1st valid IP address: 10.0.16.1 /20

• Last valid Ip address10.0.31.254 /20

• Broadcast: 10.0.31.255 /20

3rd vlan id: 10.0.32.0 /20

And so on. . .

The reason I am asking this is because I have seen many articles stating that for the network 172.16.0.0 with /30 SM the 1st valid subnet is 172.16.0.4 /30 and I was wondering why one could not use 172.16.0.0 /30 and the 2nd available subnet be 172.16.0.4 /30.

3 REPLIES
Cisco Employee

Re: IP Subnetting question

These are all ok as long as you make sure that "ip subnet-zero" is configured, which has been default for a while anyway. Without this statement, 10.0.0.1/20 would be considered invalid.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: IP Subnetting question

Could I have a router with 3 interfaces configured as followed without an IP overlapping conflict?

e0/0: 10.1.0.1 /16

e0/1: 10.1.0.1 /24 or 10.1.0.2 /24

e0/2: 10.1.0.1 /28 or 10.1.0.3 /28

IP subnet zero being enabled

Cisco Employee

Re: IP Subnetting question

You couldn't since these 3 subnets are ovelapping. The router would give you the following kind of error message:

% 10.1.0.0 overlaps with Ethernet0/0

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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