| 2501 | IOMX | IMERYS OILFIELD MINERALS INC | | | SCAC4 |
| 2502 | ION | Aire Aviation Ltd. | Callsign: Gorrion, | M | AVCODES |
| 2503 | IONA | IONIA MANAGEMENT S A | | | SCAC4 |
| 2504 | IONR | ION-TRACE INC | | | SCAC4 |
| 2505 | IONY | INDIAGEMS OF NEW YORK INC | | | SCAC4 |
| 2506 | IOP | Caliop | | F | AVCODES |
| 2507 | IOP | Caliop | Callsign: Caliop, | F | AVCODES |
| 2508 | IOPU | CANTREYN, LTD | | | SCAC3 |
| 2509 | IOPX | MidAmerican Energy Co. | added 4/1996; 7/2000-7/2005 | | NAKINA |
| 2510 | IOPX | Midwest Power Systems | added 4/1993; 1/1994; eliminated 4/1996 | | NAKINA |
| 2511 | IOPX | Iowa Power & Light Co. | added 4/1977; 1/1980; eliminated 4/1993 | | NAKINA |
| 2512 | IOPX | Iowa Power & Light Co. | 1/80; eliminated 4/93 | | NRHS |
| 2513 | IOPX | Midwest Power Systems | added 4/93; 1/94; eliminated 4/96 | | NRHS |
| 2514 | IOPX | MidAmerican Energy Co. | added 4/96; 7/2000 | | NRHS |
| 2515 | IOPX | MIDAMERICAN ENERGY CO [MIDWEST POWER SYSTEMS; IOWA POWER & LIGHT] | | | PWRC |
| 2516 | IOPX | Iowa Power and Light (MidAmerican Energy) | | | RSRV |
| 2517 | IOPZ | IOWA PUBLIC SERVICE | | | PWRC |
| 2518 | IORX | Indiahoma Refining Co. | 8/1920-7/1923 | | NAKINA |
| 2519 | IORX | *INDIAHOMA REFINING CO. | | | PWRC |
| 2520 | IORX | *INDIANA & OHIO RAILWAY - PASSENGER SVC. | | | PWRC |
| 2521 | IORY | The Indiana & Ohio Ry. | added 7/1999; 7/2000-7/2005 | | NAKINA |
| 2522 | IORY | The Indiana & Ohio Rail System | eliminated 7/1999 | | NAKINA |
| 2523 | IORY | The Indiana & Ohio Ry. Co. | added 1/1988; 1/1994 | | NAKINA |
| 2524 | IORY | The Indiana & Ohio Ry. Co. | added 1/88; 1/94 | | NRHS |
| 2525 | IORY | The Indiana & Ohio Rail System | eliminated 7/99 | | NRHS |
| 2526 | IORY | The Indiana & Ohio Ry. | added 7/99; 7/2000 | | NRHS |
| 2527 | IORY | INDIANA & OHIO RAILWAY COMPANY | | | PWRC |
| 2528 | IORY | Indiana and Ohio Railway | | | RSRV |
| 2529 | IORY | INDIANA & OHIO RAILWAY COMPANY | | | SCAC4 |
| 2530 | IORY | Indiana & Ohio Railway Company | | USA | TRAFFIC |
| 2531 | IOS | Isles of Scilly Skybus Ltd. | IATA: 5Y, Callsign: Scillonia, Designator 5Y* recalled by IATA. IATA designator changed from FW | GB | AVCODES |
| 2532 | IOS | Isles of Scilly Skybus Ltd. | IATA: FW, Callsign: Scillonia, IATA designator changed to 5Y* | GB | AVCODES |
| 2533 | IOS | Isles of Scilly Skybus Ltd. | Callsign: Scillonia, Designator 5Y* recalled by IATA. IATA designator changed from FW | GB | AVCODES |
| 2534 | IOS | Isles of Scilly Skybus | | | ICAO1 |
| 2535 | IOSC | INTEROCEAN STEAMSHIP CORPORATION, S | | | SCAC4 |
| 2536 | IOSF | IOSCO TRUCKING INC | | | SCAC4 |
| 2537 | IOSX | Interstate Oil Shippers Line | 7/1923 | | NAKINA |
| 2538 | IOSX | *INTERSTATE OIL SHIPPERS LINE | | | PWRC |
| 2539 | IOSX | *INDIANA & OHIO SCENIC RAIL CORP. | | | PWRC |
| 2540 | IOTA | IOTA | | | SCAC4 |
| 2541 | IOTC | INTERNATIONAL ORDNANCE TECHNOLOGIES | | | SCAC4 |
| 2542 | IOTG | IBISEVIC, OMER TRUCKING | | | SCAC4 |
| 2543 | IOTK | INDENT OILFIELD TRUCKING | | | SCAC4 |
| 2544 | IOTL | I O TRUCKING LLC | | | SCAC4 |
| 2545 | IOTN | INDIAN ORCHARD TRUCKING INC | | | SCAC4 |
| 2546 | IOTP | IN AND OUT TRANSPORT | | | SCAC4 |
| 2547 | IOTX | Inter Ocean Refining Co. | 8/1920-7/1923 | | NAKINA |
| 2548 | IOTX | INTER OCEAN REFINING CO. | | | PWRC |
| 2549 | IOUU | TRANS DISTANCE LINE LTD | | HK | BIC |
| 2550 | IOX | Transit Co., Ltd. | added 10/1957; 10/1957-1/1975; eliminated 10/1975; 1/1980 | | NAKINA |
| 2551 | IOX | Union Tank Car Co. | added 10/1953; 7/1954; eliminated 10/1957 | | NAKINA |
| 2552 | IOX | Transit Co., Ltd. | 1/1952; eliminated 10/1953 | | NAKINA |
| 2553 | IOX | Imperial Oil, Ltd. | 8/1920-1/1950 | | NAKINA |
| 2554 | IOX | Imperial Oil, Ltd. | Apr-38 | | NRHS |
| 2555 | IOX | Transit Co., Ltd. | 1/52; eliminated 10/53 | | NRHS |
| 2556 | IOX | Union Tank Car Co. | added 10/53; eliminated 10/57 | | NRHS |
| 2557 | IOX | Transit Co., Ltd. | added 10/57; 10/57-1/80 | | NRHS |
| 2558 | IOX | *IMPERIAL OIL, LTD. | | | PWRC |
| 2559 | IOX | *UNION TANK CAR CO. (TRANSIT CO., LTD.) | | | PWRC |
| 2560 | IOXP | INTEROCEAN EXPRESS CORPORATION | | | SCAC4 |
| 2561 | IOZL | I-ONE ZERO (I-10) LOGISTICS LLC | | | SCAC4 |
| 2562 | IP | Atyrau Aue Joly | ICAO: JOL, Callsign: Edil, IATA Prefix: 312, On 13 July 2009, Atyrau Airways was included in the List of air carriers banned in the European Union. Shortly thereafter (in October 2009), its airline license was revoked. | KZ | AVCODES |
| 2563 | IP | Inter European Airways Ltd. | ICAO: IEA, Callsign: Aspro, Merged into Airtours International Airways Ltd on 02 September 1993. | GB | AVCODES |
| 2564 | IP | Airlines of Tasmania Pty Ltd. | ICAO: ATM, Callsign: Airtas, Designator IP recalled by IATA. ICAO designator changed from XAL | AUS | AVCODES |
| 2565 | IP | Interregional Fluggesellschaft mbH | Callsign: Interregional, | D | AVCODES |
| 2566 | IP | P.T. Pelita Air Service | ICAO: PAS, Callsign: Pelita, IATA Prefix: 019, Designator 6D recalled by IATA. IATA designator changed from 6D*. Designator 6D* reinstated by IATA | RI | AVCODES |
| 2567 | IP | Unifly | Callsign: India Papa, Name changed to Unifly Srl | I | AVCODES |
| 2568 | IP | Unifly Srl | ICAO: IPP, Callsign: India Papa, Name changed from Unifly | I | AVCODES |
| 2569 | IP | Orbest Orizonia Airlines S.A. | ICAO: IWD, IATA Prefix: 560, Founded in 1998 as Iberworld. - Commenced operations on 11 April 1998. - Ceased operations on 05 May 2013. Name changed from Iberworld Airlines S.A.U. w.e.f. 01 May 2011. ORBEST is a member of the ORIZONIA CORPORACION group | E | AVCODES |
| 2570 | IP | Apsara International Air Co. Ltd. | ICAO: AQQ, Callsign: Apsara, Founded in 2013. Commenced operations on 08 October 2014. Ceased operations. | K | AVCODES |
| 2571 | IP | One Airlines S.p.A. | ICAO: ONS, Callsign: Air Dreams, Founded in July 2013. Commenced operations in October 2013. Ceased operations on 24 June 2020. | RCH | AVCODES |
| 2572 | IPA | IPEC Aviation Pty. Ltd. | Callsign: IPEC, | AUS | AVCODES |
| 2573 | IPA | IPEC Aviation | | | ICAO1 |
| 2574 | IPA | IPA Ferroviaria S.p.a. | | I | TRAFFIC |
| 2575 | IPAC | INDUSTRIAL PACKING INC | | | SCAC4 |
| 2576 | IPAK | ICEPAK INC. | | | SCAC1 |
| 2577 | IPAK | ICEPAK INC | | | SCAC4 |
| 2578 | IPAP | INTERNATIONAL PAPER | | | SCAC4 |
| 2579 | IPAU | INTELLIGENT PRIZE ARCHITECTURAL SOL | | | SCAC4 |
| 2580 | IPBX | Innophos, Inc. | 7/2005; added 4/2006 | | NAKINA |
| 2581 | IPBX | INNOPHOS, INC. | | | PWRC |
| 2582 | IPBX | INNOPHOS Inc. | | | RSRV |
| 2583 | IPBX | INNOPHOS INC | | | SCAC4 |
| 2584 | IPC | Intercontinental Pacific Airways, Inc. | IATA: IS, Callsign: Interpac, IATA designator changed to IO | RP | AVCODES |
| 2585 | IPC | Intercontinental Pacific Airways, Inc. | IATA: IO, Callsign: Interpac, IATA designator changed from IS* | RP | AVCODES |
| 2586 | IPCI | INTERNATIONAL PAPER CO ( INTERMODAL | | | SCAC4 |
| 2587 | IPCM | INTERDOM PARTNERS LLC | | | SCAC4 |
| 2588 | IPCT | ISC SHIPPING | | | SCAC4 |
| 2589 | IPCU | INDEPENDENT CONTAINER LINE | | | PWRC |
| 2590 | IPCU | Independent Container Line | | | RSRV |
| 2591 | IPCU | INLET PETROLEUM COMPANY | | | SCAC4 |
| 2592 | IPCX | International Paper Co. | 8/1920-1/1980; eliminated 10/1981; added 10/1984; 1/1994-7/2005 | | NAKINA |
| 2593 | IPCX | International Paper Co. | 4/38-1/80; eliminated 10/81; added 10/84; 1/94-7/2000 | | NRHS |
| 2594 | IPCX | INTERNATIONAL PAPER COMPANY | | | PWRC |
| 2595 | IPCX | International Paper | | | RSRV |
| 2596 | IPCX | INTERNATIONAL PAPER COMPANY | | | SCAC4 |
| 2597 | IPCZ | INTERNATIONAL PAPER CO. | | | PWRC |
| 2598 | IPDI | INTERAMERICAN PUBLIC DISTRIBUTION CORPORATION | | | SCAC1 |
| 2599 | IPEA | INTERNATIONAL POWER EXPRESS SA DE C | | | SCAC4 |
| 2600 | IPEC | INDECK POWER EQUIPMENT CO | | | SCAC4 |
