Designing the addresses: VLSM and DHCP

1What "designing the addresses" means

A provider gives you a block of addresses - a single block. Out of it must come every network of the company: production, the offices, the servers, the links between routers. Each piece must be large enough to hold the hosts, but no larger than necessary, because the rest is left for later.

2Equipment needed

3The design brief

4How it is computed, step by step

The method has four steps and none of them is skipped.

1. Add the growth to each requirement and round up. Production: 420 × 1.15 = 483.
2. Compute the host bits needed, from the inequality 2ⁿ − 2 ≥ the requirement. For 483 hosts, n = 9 (2⁹ − 2 = 510 usable), so the prefix is 32 − 9 = /23.
3. Sort in decreasing order of size and allocate in that order, starting from the first address of the block.
4. Verify that the total fits and note what has been left free.

A complete example, for the first two

Carry on by yourselves with Warehouse, Guests, Servers, Management and the three point-to-point links. Then check:

Do the computation on paper first, then compare with the widget. If the results differ, find where you went wrong - usually at the rounding or at the order of allocation. The widget is for checking, not for replacing the exercise: in the examination you will not have it.

Checking each subnet

Enter each subnet of your plan in turn and confirm the usable range and the broadcast address. Check in particular that the broadcast of one subnet is exactly 1 less than the network address of the next - if it is not, either you have left a hole, or the subnets overlap.

5The topology to implement

The addressing table - filled in before any command

The first two rows are filled in as a model. The convention: the gateway receives the first usable address of each subnet. It is a convention, not a rule - but if you follow it consistently, anybody can read your network without asking.

6The workbench

Part of the topology - R1 with Offices and Servers, R2 with Production, joined by a serial link. It is enough for you to implement the plan, configure DHCP and the relay, and provoke all four problems of section 10. The remaining segments are added in Packet Tracer, following the same pattern.

7Implementing the plan

1. Configure the routers

   The addresses in the example below are those of the plan in section 5. If your plan differs, use your own values - but then the objectives in the workbench differ as well.

   on R1copy

   Router> enable
   Router# configure terminal
   Router(config)# hostname R1
   R1(config)# no ip domain-lookup
   
   R1(config)# interface gigabitEthernet 0/0/0
   R1(config-if)# description LAN Offices
   R1(config-if)# ip address 172.20.18.1 255.255.255.0
   R1(config-if)# no shutdown
   R1(config-if)# exit
   
   R1(config)# interface gigabitEthernet 0/0/1
   R1(config-if)# description LAN Servers
   R1(config-if)# ip address 172.20.19.193 255.255.255.192
   R1(config-if)# no shutdown
   R1(config-if)# exit
   
   R1(config)# interface serial 0/1/0
   R1(config-if)# description Link towards R2
   R1(config-if)# ip address 172.20.20.33 255.255.255.252
   R1(config-if)# clock rate 128000
   R1(config-if)# no shutdown
   R1(config-if)# end
   
   R1# show ip interface brief
   

   on R2copy

   Router> enable
   Router# configure terminal
   Router(config)# hostname R2
   
   R2(config)# interface gigabitEthernet 0/0/0
   R2(config-if)# description LAN Production
   R2(config-if)# ip address 172.20.16.1 255.255.254.0
   R2(config-if)# no shutdown
   R2(config-if)# exit
   
   R2(config)# interface serial 0/1/0
   R2(config-if)# ip address 172.20.20.34 255.255.255.252
   R2(config-if)# no shutdown
   R2(config-if)# end
   

   A /23 mask is not 255.255.255.0 Production has 512 addresses, so the mask is 255.255.254.0. It is the most frequent mistake of this exercise, and it has a treacherous symptom: half the hosts answer, the other half do not. Check with show ip interface brief and with the subnet calculator above.
2. Add provisional static routing

   The two routers do not yet know each other. Routing proper is the subject of laboratory 5; for now we write the routes by hand, just enough to test the plan.

   on R1copy

   R1(config)# ip route 172.20.16.0 255.255.254.0 172.20.20.34
   

   on R2copy

   R2(config)# ip route 172.20.18.0 255.255.255.0 172.20.20.33
   R2(config)# ip route 172.20.19.192 255.255.255.192 172.20.20.33
   

   Check with show ip route on each router, then ping from the server in the Servers segment towards the interface of R2. If it works, the addressing plan is consistent.

8DHCP on the router

A Cisco router can itself be a DHCP server. It is the usual solution in small and medium networks: you no longer need a dedicated server.

1. Configure the pools
   on R1copy

   R1(config)# ip dhcp excluded-address 172.20.18.1 172.20.18.20
   R1(config)# ip dhcp excluded-address 172.20.16.1 172.20.16.20
   
   R1(config)# ip dhcp pool BIROURI
   R1(dhcp-config)# network 172.20.18.0 255.255.255.0
   R1(dhcp-config)# default-router 172.20.18.1
   R1(dhcp-config)# dns-server 172.20.19.194
   R1(dhcp-config)# lease 0 8 0
   R1(dhcp-config)# exit
   
   R1(config)# ip dhcp pool PRODUCTIE
   R1(dhcp-config)# network 172.20.16.0 255.255.254.0
   R1(dhcp-config)# default-router 172.20.16.1
   R1(dhcp-config)# dns-server 172.20.19.194
   R1(dhcp-config)# end
   
   R1# show ip dhcp pool
   

   The exclusions are always written first If you define the pool before the exclusions, the router may already have allocated the gateway address to a host. The result is an address conflict that is hard to diagnose, because it appears only when enough hosts connect.
   
   lease 0 8 0 means 0 days, 8 hours, 0 minutes. In an office network, 8 hours cover a working day; in a guest network, one hour is more suitable, so that the addresses are released quickly.
2. Test on a host

   In the workbench in this page: pick PC-BIR and press the request through DHCP button, or type ipconfig /renew in the terminal. In Packet Tracer: switch the IP configuration of the host to DHCP.

   on PC-BIRcopy

   ipconfig /release
   ipconfig /renew
   ipconfig /all
   

   This must appear:

   what should appear

   DHCP: address received from R1
   IP address......................: 172.20.18.21
   Subnet Mask.....................: 255.255.255.0
   Default Gateway.................: 172.20.18.1
   DNS Server......................: 172.20.19.194
   

   Notice the .21: the first twenty addresses were excluded, so the first free one is the twenty-first. On the router, check the record:

   on R1copy

   R1# show ip dhcp binding
   

   The address appears together with the MAC address of the client. This is the binding that makes the same host receive the same address next time as well.

3. Follow DORA in detail in Packet Tracer

   In Simulation, filtered on DHCP, run ipconfig /renew on the host and follow the four messages. For each of them, open the envelope and note:

   Message	Source IP	Destination IP	Unicast or broadcast?
   Discover
   Offer
   Request
   Ack

   Question

   What source IP address does the Discover message have, and why?

   See the answer

   0.0.0.0. The client does not yet have any address - that is precisely why it is asking. The destination is 255.255.255.255, a limited broadcast, because it does not know the address of the server either.

   The important consequence: being a broadcast, the message passes through no router. That is why, for segments without a local server, a relay is compulsory - exactly what follows.

9DHCP relay

The Production segment has no local DHCP server. The server is on R1, beyond a serial link - and the request, being a broadcast, does not get through.

1. Convince yourselves that it does not work

   On PC-PROD, press request through DHCP. You must obtain:

   what should appear

   DHCP: no server answered. The host assigned itself an APIPA address.
   IP address......................: 169.254.x.x
   Subnet Mask.....................: 255.255.0.0
   Default Gateway.................: 0.0.0.0
   

   The address that says everything A host with an address from 169.254.0.0/16 received no answer to its Discover. Do not look elsewhere: either there is no server, or the request does not reach it (missing relay, wrong VLAN, cable in the wrong port), or the pool is exhausted. It is one of the few symptoms in networking that points directly at the cause.
2. Configure the relay
   on R2, the interface facing the hostscopy

   R2(config)# interface gigabitEthernet 0/0/0
   R2(config-if)# ip helper-address 172.20.18.1
   R2(config-if)# end
   

   The address is that of the interface of R1 on which the DHCP service runs. What the router does: it takes the broadcast request, turns it into a unicast towards the server and attaches the information about the network it came from. The server picks the right pool from that information - which is why a pool for the Production network must exist on R1.

   Ask for an address on PC-PROD again. Now this must appear:

   what should appear

   DHCP: address received from R1 (through the relay)
   IP address......................: 172.20.16.21
   Subnet Mask.....................: 255.255.254.0
   Default Gateway.................: 172.20.16.1
   

   Notice the mask: 255.255.254.0, that is /23 - it comes from the PRODUCTIE pool, not from the Offices one. The server chose correctly.

10Four problems to diagnose

Provoke each situation and resolve it. Each time note which command showed you the problem.

11Assignments

• Work out the complete VLSM plan, on paper, and justify each prefix chosen
• Fill in the addressing table with all nine subnets
• Implement the plan on all three routers and check with show ip route
• Configure DHCP pools for Offices, Production, Warehouse and Guests
• Configure a relay on R2 and R3 and demonstrate that it works
• Document the four DORA messages, with source and destination addresses
• Provoke and resolve the four problems of section 10
• Compute and report: how many addresses you consumed, how many were left free, in which blocks

12Going further

13Self-check questions

14Deliverables

Keep the file: laboratory 5 starts from exactly this topology.