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Filed pursuant to Rule 433
Registration Statement No. 333-133200
June 1, 2006

FREE WRITING PROSPECTUS
Dated June 1, 2006
  

9,000,000 Shares

GRAPHIC

Common Stock

This free writing prospectus relates only to the shares of common stock described below and should be read together with the preliminary prospectus supplement, dated May 19, 2006, (including the base prospectus dated May 5, 2006 and the documents incorporated by reference therein) relating to these shares of common stock.

We are offering 4,000,000 shares of our common stock to the public. Certain of our stockholders identified in the preliminary prospectus supplement are offering 5,000,000 shares of our common stock to the public. We will pay certain expenses of the selling stockholders.

Our common stock is traded on the Nasdaq National Market under the symbol "CCOI." The last reported sale price of our common stock on June 1, 2006 was $9.30 per share.

 
   Per Share
   Total
Public offering price   $ 9.0000   $ 81,000,000
Underwriting discounts     0.4725     4,252,500
Proceeds to us (before expenses)     8.5275     34,110,000
Proceeds to selling stockholders (before expenses)     8.5275     42,637,500

We and certain of our stockholders, including our Chairman and Chief Executive Officer Dave Schaeffer, have granted the underwriters a 30-day option to purchase up to an additional 1,350,000 shares on the same terms and conditions as set forth above if the underwriters sell more than 9,000,000 of firm shares of common stock in this offering.

The issuer has filed a registration statement (including a prospectus) with the Securities and Exchange Commission or SEC for the offering to which this communication relates. Before you invest, you should read the prospectus in that registration statement and other documents the issuer has filed with the SEC, for more complete information about the issuer and this offering. You may get these documents for free by visiting EDGAR on the SEC Web site at www.sec.gov. Alternatively, the issuer, any underwriter or any dealer participating in the offering will arrange to send you the prospectus and the preliminary prospectus supplement if you request it by calling toll-free 1-888-603-5847.

Joint Book-Running Managers

LEHMAN BROTHERS BEAR, STEARNS & CO. INC.

Joint Lead Manager

THOMAS WEISEL PARTNERS LLC

WACHOVIA SECURITIES FRIEDMAN BILLINGS RAMSEY


SUMMARY CONSOLIDATED FINANCIAL AND OTHER DATA

        The following summary historical financial information should be read in conjunction with our consolidated financial statements and the related notes included in the preliminary prospectus supplement. The "As Adjusted" column in the Balance Sheet Data gives effect to this offering as if it had occurred on March 31, 2006 and based on the net proceeds as described under "Use of Proceeds."

 
   March 31, 2006
 
   Actual
   As Adjusted
 
   (in thousands)
Balance Sheet Data:            
Cash and cash equivalents and short-term investments—restricted   $ 21,241   $ 54,651
Working capital     11,055     44,465
Property and equipment, net     284,474     284,474
Total assets     333,118     366,528
Capital lease obligations     89,352     89,352
Long term notes payable (net of discount of $2,983)     7,208     7,208
Stockholders' equity     208,161     241,571

2


USE OF PROCEEDS

        We estimate that we will receive net proceeds from this offering of approximately $33.4 million, after deducting underwriting discounts and estimated expenses. We intend to use the net proceeds to fund the expansion of our sales and marketing efforts, to connect additional buildings to our network and for general corporate purposes, which may include potential acquisitions of complementary businesses. We will receive no proceeds from the sale of shares by the selling stockholders.

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CAPITALIZATION

        The following table sets forth our cash, cash equivalents and short-term investments and our consolidated capitalization as of March 31, 2006:

        You should read this table in conjunction with our consolidated financial statements and the related notes included in the preliminary prospectus supplement.

 
   As of March 31, 2006
  
 
   Actual
   As Adjusted
  
 
   (unaudited)

  
 
   (in thousands)

  
Cash, cash equivalents and short-term investments—$630 restricted   $ 21,241   $ 54,651  
   
 
 
Debt (including current maturities):              
  Capital lease obligations     89,352     89,352  
  Convertible Subordinated Notes (net of discount of $2,983)     7,208     7,208  
   
 
 
      Total debt     96,560     96,560  
   
 
 
Stockholders' equity:              
  Common stock, par value $0.001 per share; 75,000,000 shares authorized; 44,128,879 shares outstanding; 48,128,879 outstanding as adjusted     44     48  
  Additional paid-in capital     434,318     467,724  
  Stock purchase warrants     764     764  
  Accumulated other comprehensive income     768     768  
  Treasury stock, 61,462 shares     (90 )   (90 )
  Accumulated deficit     (227,643 )   (227,643 )
   
 
 
      Total stockholders' equity     208,161     241,571  
   
 
 
Total capitalization   $ 304,721   $ 338,131  
   
 
 

The tables and calculations above exclude:

4


DILUTION

        Dilution is the amount by which the offering price paid by the purchasers of the common stock to be sold in this offering exceeds the net tangible book value per share of common stock after this offering. The net tangible book value per share is determined at any date by subtracting our total liabilities from the total book value of our tangible assets (total assets less intangible assets) and dividing the difference by the number of shares of our common stock outstanding at that date.

        Our net tangible book value as of March 31, 2006 was $206.0 million, or $4.67 per share. After giving effect to the receipt of approximately $33.4 million of estimated net proceeds from our sale of 4,000,000 million shares of common stock in this offering, our as adjusted net tangible book value as of March 31, 2006 would have been approximately $239.4 million, or $4.97 per share. This represents an immediate increase in net tangible book value of $0.30 per share to existing shareholders and an immediate dilution of $4.03 per share to new investors purchasing shares of our common stock in this offering. The following table illustrates this substantial and immediate per share dilution to new investors:

Offering price per share         $ 9.00
  Net tangible book value before the offering   $ 4.67      
  Increase per share attributable to investors in the offering     0.30      
   
     
As adjusted net tangible book value after the offering           4.97
         
Dilution per share to new investors         $ 4.03
         

        The following table summarizes as of March 31, 2006:


 
   Shares Purchased
   Total Consideration
    
 
   Average
Price Per
Share
 
   Number
   Percent
   Amount
   Percent
 
   (in thousands)
Pre-offering stockholders   44,128,879   91.7 % $ 364,296   91.0 % $ 8.26
Investors in the offering   4,000,000   8.3 %   36,000   9.0 %   9.00
   
 
 
 
 
  Total   48,128,879   100.0 %   400,296   100.0 %   8.32
   
 
 
 
 

The tables and calculations above exclude:

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SELLING STOCKHOLDERS

        The following table supplements and amends the information set forth under the caption "Selling Stockholders" in the preliminary prospectus supplement.

 
   Shares Beneficially Owned
Before Offering
   Number of
Shares Being
Offered for Resale
under this
Prospectus
   Shares Beneficially Owned
After Offering
  
Name
  
  Number
   Percentage
   Number
   Percentage
  
Entities affiliated with BNP-Europe Telecom & Media Fund II, LP (1)   4,277,741   9.7 % 388,283   3,889,458   8.1 %
Jean Jacques Bertrand   3,750   *   340   3,410   *  
Entities affiliated with Boulder Ventures IV, LP (2)   709,817   1.6 % 354,909   354,908   *  
Cisco Systems Capital Corporation (3)   1,278,745   2.9 % 1,278,745   —   —  
Columbia Ventures Corporation (4)   3,099,814   7.0 % —   3,099,814   6.4 %
Entities affiliated with Jerusalem Venture Partners (5)   5,706,565   12.9 % 899,410   4,807,155   10.0 %
Erel Margalit   3,750   *   591   3,159   *  
Entities affiliated with Nassau Capital Partners IV, LP (6)   1,041,241   2.4 % 520,621   520,620   1.1 %
Entities affiliated with Oak Investment Partners IX, LP (7)   4,739,991   10.7 % 900,000   3,839,991   8.0 %
Entities affiliated with Worldview Technology Partners (8)   3,460,253   7.8 % 657,101   2,803,152   5.8 %
Dave Schaeffer (9)(10)   1,792,886   4.1 % —   1,792,886   3.7 %
*
Denotes less than 1% ownership

(1)
Includes shares held by entities affiliated with BNP-Europe Telecom & Media Fund II, LP including: BNP Europe Telecom & Media Fund II, LP (4,045,256 shares of common stock, of which 367,181 are being offered hereby or 404,204 if the over-allotment option is exercised in full) and Natio Vie Development3 (232,485 shares of common stock, of which 21,102 are being offered hereby or 23,230 if the over-allotment option is exercised in full).

(2)
Includes shares held by entities affiliated with Boulder Ventures IV, LP including: Boulder Ventures IV, LP (42,866 shares of common stock, of which 21,433 are being offered hereby) and Boulder Ventures IV (Annex), LP (666,951 shares of common stock, of which 333,476 are being offered hereby).

(3)
Cisco Systems Capital Corporation is a wholly-owned subsidiary of Cisco Systems, Inc.

(4)
If the over-allotment option is exercised in full, 210,849 of these shares will be sold resulting in ownership of 2,888,965 shares, or 6.0%, after the offering.

(5)
Includes shares held by entities affiliated with Jerusalem Venture Partners including: Jerusalem Venture Partners III (Israel), L.P. (63,741 shares of common stock, of which 10,046 are being offered hereby), Jerusalem Venture Partners Entrepreneurs Fund III, L.P. (174,847 shares of common stock, of which 27,558 are being offered hereby), Jerusalem Venture Partners Entrepreneurs Fund IV, L.P. (27,499 shares of common stock, of which 4,334 are being offered hereby), Jerualem Venture Partners IV (Israel), L.P. (73,985 shares of common stock, of which 11,661 are being offered hereby), Jerusalem Venture Partners III, L.P. (2,274,504 shares of common stock, of which 358,483 are being offered hereby), Jerusalem Venture Partners IV, L.P. (3,065,953 shares of common stock, of which 483,224 are being offered hereby) and Jerusalem Venture Partners IV-A, L.P. (26,036 shares of common stock, of which 4,104 are being offered hereby).

(6)
Includes shares held by entities affiliated with Nassau Capital Partners IV, LP including: Nassau Capital Partners IV, LP (1,025,143 shares of common stock, of which 512,572 are being offered hereby) and NAS Partners I, LLC (16,098 shares of common stock, of which 8,049 are being offered hereby).

(7)
Includes shares held by entities affiliated with Oak Investment Partners IX, LP including: Oak Investment Partners IX, LP (4,581,346 shares of common stock, of which 869,877 are being offered hereby), Oak IX Affiliates Fund, LP (49,499 shares of common stock, of which 9,399 are being offered hereby) and Oak IX Affiliates (Annex), LP (109,146 shares of common stock, of which 20,724 are being offered hereby).

(8)
Includes shares held by entities affiliated with Worldview Technology Partners including: Worldview Technology Partners III, L.P. (1,758,667 shares of common stock, of which 333,971 are being offered hereby); Worldview Technology International III, L.P. (433,464 shares of common stock, of which 82,315 are being offered hereby), Worldview Strategic Partners III, L.P. (38,865 shares of common stock, of which 7,380 are being offered hereby), Worldview III Carrier Fund, L.P. (96,605 shares of common stock, of which 18,345 are being offered hereby), Worldview Technology Partners IV, L.P. (966,906 shares of common stock, of which 183,615 are being offered hereby), Worldview Strategic Partners IV, L.P. (8,056 shares of common stock, of which 1,530 are being offered hereby) and Worldview Technology International IV, L.P. (157,690 shares of common stock, of which 29,945 are being offered hereby).

(9)
If the over-allotment option is exercised in full, 750,000 of these shares will be sold resulting in ownership of 1,042,886 shares, or 2.2% after the offering.

(10)
Shares beneficially owned does not include 71,958 restricted shares vesting after July 1, 2006, and options for 576,930 shares of common stock that vest on November 1, 2006.

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SUMMARY CONSOLIDATED FINANCIAL AND OTHER DATA
USE OF PROCEEDS
CAPITALIZATION
DILUTION
SELLING STOCKHOLDERS