--- title: "Design network for two buildings with fiber" subject: "Internet and Intranet" chapter: 5 key: "semester_8/internet_and_intranet:network-design-guidelines-two-buildings" source: https://www.bctengineer.com/q/semester_8/internet_and_intranet/network-design-guidelines-two-buildings site: BCT Engineering --- # Design network for two buildings with fiber Internet and Intranet · Chapter 5: Designing Internet Systems and Servers - Asked in: 2073 Bhadra, 2+10 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/internet_and_intranet/network-design-guidelines-two-buildings - Whole chapter: https://www.bctengineer.com/md/semester_8/internet_and_intranet/chapter-5 ## Question What are the guidelines to design proper network system? You are asked to design a Network System for the organization which has 2 different buildings at less than 500 meter distance. The main building is 5-storey building which has Network/Server room in third floor. There are 3 computer training labs in 1st, 2nd and 3rd floor consisting of 20 computers at each lab. You need to provide internet to all lab computers as well as office computers for staffs. Connect two buildings from Fiber network. You can suggest multiple numbers of Wireless AP and/or Wired Network systems for your requirement. Prepare the required technical documents and suggest a network for the organization. ## Short answer Design using hierarchical model with core switch in server room, access switches per floor, VLANs for labs/staff/wireless, and single-mode fiber between buildings. Provide CAT6 cabling, PoE access switches, seven wireless APs, DHCP/DNS servers, and a router/firewall. Use private IP subnets with inter-VLAN routing on the core L3 switch. ## Answer #### Guidelines for designing a proper network 1. Find the requirements first: users, applications, traffic, growth for 3-5 years. 2. Use a **hierarchical** model (core, distribution, access) so the network is easy to scale and troubleshoot. 3. Follow structured cabling standards (TIA/EIA-568): CAT6 up to 90 m for permanent link, fibre for longer or between buildings. 4. Plan **redundancy** and power backup (UPS) for critical links and servers. 5. Plan IP addressing and VLANs; keep departments in separate subnets. 6. Build in security (firewall, ACL, wireless encryption) and manageability (monitoring, labelling, documentation). 7. Balance cost against performance, and leave spare ports and rack space. #### Assumptions - Building A (main, 5 storeys) has the Network/Server room on the 3rd floor. Building B (annex) is about 450 m away, with 2 floors. - Labs: 3 labs x 20 PCs on floors 1, 2 and 3 of Building A = 60 PCs. - Staff offices: 30 PCs in Building A (6 per floor) and 20 PCs in Building B. - One Wi-Fi AP per floor in A (5) and 2 in B. One ISP link of 100 Mbps. - Floor height 3.5 m. Average horizontal cable run 30 m (A) and 25 m (B). #### Topology ```text Internet (ISP) | [Router + Firewall] | +----[Core L3 Switch]----+ 3rd floor | Servers: DHCP, | Server room | DNS, Proxy, Web | | | OM3 riser fibre OS2 fibre ~500 m | | [SW-F1][SW-F2][SW-F4][SW-F5] [SW-B] Lab1 Lab2 Staff Staff Bldg B +Staff +Staff +AP +AP Staff +AP +AP +2 AP (SW-F3 sits in the server room itself) ``` Single-mode (OS2) armoured fibre with 1000BASE-LX SFP is laid in an underground HDPE duct between the buildings. Single-mode is chosen because it supports future 10G over the full distance. #### IP plan (private range 192.168.0.0/16) | Network | Subnet | Usable hosts | Gateway | VLAN | |---|---|---|---|---| | Lab 1 (20 PCs) | 192.168.10.0/27 | 30 | .1 | 10 | | Lab 2 | 192.168.10.32/27 | 30 | .33 | 20 | | Lab 3 | 192.168.10.64/27 | 30 | .65 | 30 | | Staff Building B | 192.168.10.96/27 | 30 | .97 | 40 | | Staff Building A (30 PCs) | 192.168.10.128/26 | 62 | .129 | 50 | | Wi-Fi (all APs) | 192.168.20.0/24 | 254 | .1 | 60 | | Servers | 192.168.30.0/27 | 30 | .1 | 70 | | Management | 192.168.99.0/27 | 30 | .1 | 99 | A /27 is enough for a 20-PC lab (20 PCs + gateway + spare). DHCP on the server gives addresses; servers and APs use static addresses. Inter-VLAN routing is done on the core L3 switch. #### Bill of quantities | Item | Specification | Qty | |---|---|---| | UTP cable | CAT6, 305 m box (about 3,400 m + 10% wastage) | 13 boxes | | Information outlet | Single-port faceplate with CAT6 jack (117 + spare) | 120 | | Patch panel | 24-port CAT6, rack mount | 9 | | Patch cords | CAT6, 2 m (user side) | 117 | | Patch cords | CAT6, 1 m (panel to switch) | 117 | | Core switch | L3, 24 x GE + 4 x SFP+ | 1 | | Access switch | 48-port PoE, 2 x SFP (floors 1-3) | 3 | | Access switch | 24-port PoE, 2 x SFP (floors 4, 5 and Building B) | 3 | | Wireless AP | Dual-band, PoE | 7 | | Fibre OS2 | 6-core armoured, outdoor | 600 m | | Fibre OM3 | 6-core indoor riser | 150 m | | SFP modules | 1G LX (single-mode) / SX (multimode) | 4 / 8 | | Router + firewall | With NAT, ACL | 1 | | Servers | DHCP, DNS, proxy/web | 2 | | Racks | 42U (server room), 9U wall mount (floors) | 1 / 5 | | UPS | 3 kVA online (server room), 1 kVA (floors) | 1 / 5 | | Misc | Conduit, trunking, labels, tester | lot | #### Technical documents to prepare - Requirement and scope document, assumptions. - Logical and physical network diagram (as above), floor plans with outlet positions. - IP addressing and VLAN plan, DHCP scope sheet. - Cable schedule and labelling scheme, fibre test results. - Bill of quantities with cost estimate. - Configuration backup, security policy, user manual, and as-built drawing after installation. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why use a hierarchical model (core, distribution, access)? A. It reduces the need for cabling B. It makes the network easy to scale and troubleshoot C. It removes the need for VLANs D. It avoids using switches Answer: B. The layered design aids scaling and troubleshooting. (Key point: Hierarchical model) 2. What is the permanent-link limit for CAT6 horizontal cable in the answer? A. 50 m B. 150 m C. 500 m D. 90 m Answer: D. CAT6 permanent link goes up to 90 m; fibre is used for longer runs or between buildings. (Key point: Cabling limit) 3. Why is single-mode (OS2) fibre chosen between the buildings? A. It is cheaper than CAT6 B. It supports future 10G over the full distance C. It needs no SFP modules D. It works only indoors Answer: B. Single-mode carries 10G over the ~500 m distance, giving room for growth. (Key point: Fibre choice) 4. Why is a /27 sufficient for a 20-PC lab? A. It has 30 usable hosts, enough for 20 PCs, gateway and spare B. It has 254 usable hosts C. It supports only 20 hosts exactly D. It is required by DHCP Answer: A. A /27 gives 30 usable addresses. 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